i been buying up weapond mods like scopes extended ammo clips but i can not figure how to attach them to the weaponds any clues?

This was hard for me, too. Click on the weapon and hit 'x' it opens the menu.

Go over the weapon you want to improve, and press LB (360) press X (PC) im too sure on the PS3 though. A list should appear.

Oh damn, Skooma. I never considered they weren't on a PC. Thanks for the backup.

On the PS3 it's L2