Make sure the small spring arm is not bound (a little lube) - push the spring arm into the body of the mirror at the corner of the mounting rectangle and in a twisting motion slide the other side into the mounting space.
Hope this helps as the first couple times you do it it's a buggar - but once you get the motion down it's not too bad
Yeah, it actually goes on well. It's about getting the right angle/leverage, and it will just screw right in.