I installed new rotors on my 4x2 last weekend. I don't know if there's a difference between the 4x2 and the 4x4 grease caps - and your profile doesn't say which one you have - so I'll take a stab at answering your question.
I used a rubber mallet to tap it gently in place - all around the rounded outer edge - until it lined up straight in the hub. Then I gave it more forceful whacks with the mallet until it was seated all the way in. If you're talking about the inner grease seal, the same technique applies.
I actually had more trouble getting the grease cap out (without damage), than back in. The next time I have to remove the grease cap, I may just buy a couple of new ones first, then beat the crap out of the old ones to get them out. Rich Lunder told me (after I had already finished the job) that he uses a large pair of channel-lock pliers to get them out. That may be the best way to remove them without having to buy a special tool.