You do not. Unless...
This is not realistic.
However... see addendum for some hand-waving.
Ok, so Polonium-210 has a really high LD-50 value, that is all fine and well.
But where are you going to get it?
Your first problem is that Polonium is an extremely rare substance because it can only be created in nuclear reactors.
The next problem that it has a half-life of only 138 days, meaning that even if you have a phenomenally high production rate of 1 kg per 138 days, you can only ever have 2 kg available. The current world production of Polonium is 0.1 kg per annum.
Another big problem you have is that Polonium-210 is an alpha emitter. An alpha emitter must be ingested to do any actual harm. So you must get people to breathe or eat the Polonium.
That turns your task into a problem of logistics. And this is not some fun exotic little scheme you can just hand-wave. A few years ago, someone described the problem of logistics and getting stuff to people like this:
Suppose the cure for AIDS is a glass of water. All we need to do is deliver from our place, a glass of water, to every person with AIDS. How many can we save?
Answer: only about 50%. The rest will have died before you get the water to them.
One more problem — as brought up by WhatRoughBeast — is that Polonium-210 generates lots of heat. A single gram generates heat like two old-school 70W light bulbs, in a volume of a mere tenth of a cubic centimeter. And since you need to keep this material properly shielded and sealed while transporting it to your dispersal sites, you have a problem trying to keep this from setting fire to your transportation device.
Yet another problem you have is that this LD-50 amount only counts for an instant dose. A dose that is stretched out in time immediately raises the amount of Polonium that is needed.
And your final — huge — problem is: people are watching out for it. Radiological watchdogs all around the Earth will instantly pick up on this as soon as it gets out and sound the alarm.
These things put together: that they must — in an impossibly short time span — produce, distribute, disperse and make people ingest this substance, all while not being noticed, means that your evil dictator is faced with an impossible task.
Reading on the same page that you linked to, you have a much better candidate: the botulin toxin.
The Clostridium Botulinum bacterium is everywhere. You can get it from plain soil if you want to. The botulin toxin even has everyday consumer applications, such as being the active ingredient in a Botox treatment.
If your dictator is going to be a prestigious twit that must go for the highest LD-50 value of all, then their plan will fail. Sorry, but it just will.
Suppose we solve the production problem. Let us say that the newly discovered mother lode of element 118 — Unobtainium — conveniently found only in your mad dictator's country, will turn to Polonium-210 when subjected to some really intense hand-having. Your dictator now has an unlimited amount of Po-210 available.
Let us then go for maximum damage with least amount of notice. How to deliver a radioactive substance as close to people as possible, with least amount of notice, and as distributed as possible. How would you do it?
Smoke detectors contain Americium-241, a radioactive substance. It is an essential part that makes them work. No-one would notice if that is replaced with a sample Po-210, in the form of nanometer-sized aerosol particles instead. Po-210 also has a really convenient decay chain: it turns to Pb-206, a stable isotope of Lead, meaning there will be no leakage of tell-tale decay products.
Then on a given signal, or at a given time, the little explosive charge present in all the smoke detectors go off at once. In every home, office, warehouse, hospital and other places where Mad Dictator Inc. have delivered their smoke detectors, you have contaminated the place with dust clouds of Polonium-210.