Sitting here with the rifle and pin in front of me, and am still not sure how to replace it. It's the rear push pin. How do I take out the one that's in there, and get another one back in.

It doesn't seem like it should be that hard, could somebody please explain it to me?

you have to take off buttstock. there is a spring and detent pin in the back of the receiver. Be careful not to lose. The detent pin holds the push pin in place. When they are out you can simply replace the push pin with groove towards buttstock, then slide detent pin in with spring behind it and replace buttstock. When replacing buttstock you should be able to leave buffer tube in place so that buffer detent and spring are not lost.
An excellent resource is the USMC M16 manual. Mine cost $5-6 and was well worth it.

Hey, just the info Ineeded, thanks!