steyr "drop in" picatinny rail Installation?

[45er]

I have a steyr picatinny rail for my Steyr USR. It is supposed to replace the carrying handle/scope on my rifle. The dealer told me to use the nose of a 223 cartridge to depress the button towards the front of the carrying handle/scope, move it forward and it would come off, thus allowing the installation of the picatinny rail. When I try this the CH/S will not budge. Does anybody know where I am screwing up?
MANY THANKS