There are a number of problems, lets solve them one at a time:
Having a planet in the first place:
If the planet was captured by the Magnetar after it was formed then that would explain it surviving the supernova. You would need some interesting interactions in order to explain a stable non-elliptical orbit but it's theoretically possible for capture to happen.
Providing light and heat:
The Magnetar won't do it. You have two choices here:
- Internal heating of the planet by gravitational or magnetic forces - this would give you an ice crust with liquid water beneath.
- A binary system with another star and the Magnetar orbiting that other star. Your planet could orbit the Magnetar or orbit the combined binary. Either way you would get temperature variations depending on your distance from the light star.
Note that Magnetar research has suggested that a binary pair may be needed for Magnetar to form in the first place - or that it would help them form anyway.
Not getting ripped to shreds by the magnetic field
This is hard to answer, but remember that the strength of the field varies for different Magnetars and also that the further you are from the Magnetar the weaker the field will become. In other words you just need to move the planet out until it is far enough from the star to survive.
Traveling rocks that are naturally magnetic
It depends what you mean by travelling, magnetic rocks could plausibly be pulled along the ground or even act like tides on an ocean under the influence of the field.
possibly floating landmasses, depending on the strength of the field
No. Sorry. That would never be stable. They would either fly up into the air or fall to the ground.
an animal-type species that has perfect sense of location due to magnetic sensitivity
Highly unlikely - since magnetic sensitivity would just let it sense where the Magnetar is, and the planet is constantly moving through the agnetars field.
travel via magnetic ships that ride the field.
Slightly more plausible than floating land since you can have something actively working to stabilize them. It still doesn't really make sense though. You have a powerful magnetic field but the planet is already moving through that at massive speed. You could probably generate electricity off it though...
Your how-else question might be covered here:
What could cause rock formations (small stones, boulders, islands...) to levitate?