Walther SP22 question

[emsguy]

I bought a Walther SP22 M2 and want to mount the rails on top so that I can mount a red dot sight. Has anyone done this? I tried to slide the rail into the grooves in the top of the housing (it doesn't seem right to call it a slide because it doesn't move when the gun is fired). It won't go in all the way. It keeps getting stuck halfway in.

The rail seems to be made of plastic, and the housing is aluminum, so I am afraid to use any kind of force.

Any ideas?

Thanks