i built my first lower tonight.  well, i thought i've seen and played with these things enough that i thought i pretty much new what i was doing.  to make a long story short, i broke off one side of the trigger guard housing after using a hammer to punch it through.  now, i have the piece connected together with the pin (it's pretty tight, but i want to make sure it doesn't come out)...is there any special adhesive i can use to glue this piece back to the lower? or i pretty much ruined this lower?

Don't know how severe your break is.  My best guess, if the receiver isn't cracked further up you may be able to JB Weld the broken part back in to make it work.  Basically, the lower primarily just holds the 'trigger group' for lack of a better description.  Maybe if you clean your parts very well, you may get the JB Weld to set up well enough to just be a barely visible hairline, which you could paint or black in later.  This is just my best guess.
For future reference, I find that when installing the trigger guard I end up using a roll of masking tape (as a makeshift bench block) and a couple of old towels to support the lower.  And tap lightly, letting it go in on it's own.

I just posted in another thread on the need to make sure to support the receiver very well when doing this, and to grease up the pin really good.  Sorry to hear you broke yours.  

Can you post a pic, by any chance?  

JB Weld is probably the best bet...unless there's another sort of super epoxy type stuff you could get down into the crack...assuming the crack isn't too long.