I'm having some trouble mating my stag arms lower and dpms upper.  They just dont want to seem to go together very easily.  Am i missing something here?  Is there some sort of trick to getting that rear takedown pin in?

Work with them a bit, they will loosen up and bed in nicely, particularly when you start shooting it.  If they were too easy a fit, they would get sloppy after a while, which could adversely affect accuracy.

Don't force it too much--some uppers and some lowers are really too tight to fit together. And it's not seen often, but you should know that it's possible to break a lower at the pivot surfaces, so don't crank it down too hard. Can you give us some more details about what happens when you attempt to assemble the two? How much of a gap are you seeing at the rear when you try to close it?

Since the Stag lower is made by the same outfit, Continental Machine, that makes the Rock River, Century, and some other brands. They all seem to have tight rear ends (no pun intended).  We've had a number of posts on the Rock River types, and I have personally experienced the situation.  Had to use a rubber hammer to get a recent Rock River lower to accept a different brand upper; but as the others have posted, it eases up after several trys.