G5 Mouse 3 Button Problem

Just bought a g5 and have installed it with the disk and all that ... but whenever i go in game and use mouse 3 (to attach rifle) it sought of comes off et whilst showing a zoom button. When i assign mouse 3 to alternate weapon, it does not allow it to come up i.e via controls. Also this means i cant shoot and Mouse 3 does not assign. Any ideas on what is needed to do to fix this problem? Thanks