Your title is a little misleading, the part you refer to is the "take down lever" or "locking block", calling it a "pin" is confusing, a least to me.
Even looking at the picture, I could not decide what your problem was, but then I'm a little slow at times.
But you have received good advice as to how to get it into place.
Let's see the rest of the luger when you get it together!