Right click the scope/silencer and if they're compatible with the rifle/pistol you currently have euqipped, you've be given an option to mount it on the pistol/rifle. Some rifles can't have scopes or silencers, some pistols can't accept silencers. Its a case of fiding the right combination. My favourite is the LR300 with an underslung M203 and Suppressor (haven't tried adding a SUSAT, that'd be awesome).