How do you get a combat arms weapon mod

If you are talking about modifying your weapons in Combat Arms, at the top of your screen, there is a button you can click "Modification". On the left will be the list of guns you have, click on the one you want to modify, then in the box on the bottom right double click the modification you want (as long as you are the rank required to buy it, if any). It will ask if you want to attach it to the gun, click attach and there you go. If you clicked storage, then in the box to the right (above the list of modifications available for purchase), will be the list of modifications you currently have. Simply double click on the one you have just bought and it will attach itself to the weapon. NOTE that modification pieces for different gun classes will not attach to ones of other classes. For example, if you buy a silencer for a AR (Assault Rifle), you can attach it to any other AR but not to another class such as SMG (Sub-Machine Gun). Hope that helps!