You need to remove the cover, remove the spark plug (makes engine turn easier), then start the chain as far up on the sprocket as you can. Turn the sprocket by hand or use a tool to assist until you have it all the way on. Good luck!!
If I remember right, I used a 3/4" socket. With the spark plug out the engine turned over pretty easy.
If your spark plug is removed and the engine still doesn't turn over easily, then it suggests that something else is wrong. I hope not, but you might want to look into it.