Are you pushing the mag release button in with a punch or other device, to fully thread the button it? You have to do that.
If you just try to thread the button onto the end of the threaded portion by hand, and stop there (when you hit the right side of the receiver), you'll never get it threaded on where it needs to be - you need to push that button in, and spin the mag release on the left side of the lower, in order to thread that thing in there...