Pretty much everything you need is right there on that page. Here's a breakdown that largely re-digests your link, though I'll toss in a few extra tidbits to round it out a bit more.
This is actually the easiest bit. Space is big. Very big. Even our "little corner" of the universe is mind-meltingly gigantic. It would be pretty trivial for a major asteroid to sneak up on us and nobody notice.
If you want to make it even more likely to escape notice, it can come at us from the direction of the sun, where the sun itself would all but literally blind us to the oncoming threat. This would most likely have to be a retrograde orbit, which while uncommon/rare is not unheard of; naturally, ~6 months prior to the impact we'd be on the asteroid's side of the sun and could thus see it at night, but it'd be much further out and thus still more likely to be missed.
Another option is to put it on an unusually inclined orbit. Almost every object in our solar system orbits on or very close to a common plane. As a result, anyone looking for things in our solar system pretty much trains their telescopes to this plane. Anything coming at us from above or below is very likely to never be spotted at all.
One final option (well, I'm sure there's more out there, but this is the last I'm offering) would be an asteroid that looks like it won't be able to make it through the atmosphere, but have a surprising property that lets it punch all the way through. A comet hiding a dense iron core, for example. Under this scenario you could have it spotted, but all the experts telling everyone not to worry. Then BAM!
Depends on what it's made of. A dense iron core can be as little as 3km across and still cause "winter". A "softer" rocky meteor has to be bigger to survive punching through the atmosphere.
Really, for the purposes of fiction, pick any size between 3km and 10km (the size of the meteor that wiped out the dinosaurs) and call it a "rocky meteor", and you're good to go. There's so much variation even with similar compositions that you've got almost unlimited free reign here.
Short version: Anywhere, depending on how massive and how dense it is.
For best results, land or shallow water will give you the most ejection of fine particulates, which is what will block the sun and give you your "winter". Obviously anything too close to the impact location will be vaporized, and for a good ways after that "merely" pulverized. These distances depend entirely upon what the meteor's made of and how big it is.
For a minimum of direct casualties, a larger object hitting in the middle of the ocean will still give you enough ejecta to block the sun and plunge the world into winter. Obviously coastal areas will be hit hard by tidal waves, but those are infinitely more survivable than the direct blast of a meteor impact.
For the Eastern US to survive the initial impact, put your meteor nearly anywhere else. An entirely separate continent would insulate the area against tidal waves and firestorms from the impact. Even as close as the Western US and the Eastern would likely still be around to enjoy the ensuing winter. You're best bet though is to have it hit in Asia or Australia; Europe or Africa would work too, but you might get tidal waves from these. Ditto the northern bits of South America. Antarctica would be an excellent choice because the ice there would be vaporized instantly on impact, adding water vapor to the atmosphere to further block out the sun; on the other hand, you'd also get so much ice calving that you could easily see tidal waves again, but I wouldn't expect these to be as devastating by the time they reached the Eastern US.
As the meteor crashes through the atmosphere, it's going to start burning. This will be visible across huge swathes of the world; if at night, it will dramatically light up the sky. Pieces will break off as the atmosphere tries desperately to stop it, and you'll get smaller impacts all around and along its path through the air (as well as numerous fire trails that just end as the smaller bits burn up or explode).
When it hits the ground, there will be an explosion beyond anything you can comprehend, or really even hope to adequately describe. Hiroshima would look like a firecracker by comparison. There'd be an immense fireball, followed by a mushroom cloud climbing far up into the atmosphere; it would literally spread to cover the entire globe. Trees and other combustible material for many, many, many kilometers around will burst into flame as firestorms explode outward; this smoke will further contribute to the dark cloud blotting out the sun. Ground Zero will be a massive crater, and everything for another real good ways will be flattened if not completely pulverized. It will be felt all over the planet.
Larger debris ejected into the air will come crashing back down for secondary impacts all over the hemisphere; a large enough impact will send some of these on suborbital trajectories and hit the opposite side of the globe as well. None of these will come even close to the scope of the main impact though.
The effects would be quite rapid. The cloud of dust and debris and smoke will race rapidly across the entire globe, and plunge the whole planet into a reddish darkness. Temperatures will immediately start to plummet, and average ground temps will drop by around 13C (23F, if I did the math right). Not overnight, mind you, but pretty quickly; it will start off fast and slow down as the new "normal" stabilizes. Enough firestorms can increase this drop by another few degrees, but you're not realistically going to see (formerly-)sunny Miami freeze solid overnight. Still, it's enough that plenty of people will be tempted to migrate -- not that the conditions will be a whole lot better (other than warmer) further south.
Over time food supplies will dwindle, as crops die and agriculture comes to an almost instant crash. The social impacts -- riots of panicking people leading to completely collapse of order in many areas -- will be almost immediate.
Direct loss of food as most life across the planet faces extinction will cause up to 25% of the human population to starve to death (apparently we're quite smart or something). The ensuing social unrest and collapse of civilization in many areas will lead to even more deaths.
Within about a year enough of the particulates will have cleared from the air for global temperatures to have rebounded by almost half of what they fell by. You'll have similar "half-life" gains over subsequent years. Agriculture will start to be viable again around this time, but at nowhere near as productive of levels as it is currently. This, too, will improve over subsequent years.