i have mod but i cant seem to put it on using a workbench or reloading bench, so what do you use?

Go to the weapon/item you have the mod for and select it in menu. Then, hit "X". It will ask you if that is what you want modded.

This is in your Pipboy menu, not the bench.

Hey I think you just go in through the pip boy...there should be a mod option near the repair option by the weapon you try to mod.