Not sure exactly how to describe this other than linking to the feature request I made for CBA that apparently isn't possible due to this issue.
https://github.com/CBATeam/CBA_A3/issues/581
Basically it appears you can't assign mouse or joypad input as controls for mods.
Try to assign a mouse button as a keybind in CBA's "configure addons" section of the controls configuration.
@Alwin I take it this isn't going to ever be implemented? Would be quite useful for a lot of mods. CBA managed to find a workaround but there are other mods that aren't able to assign mouse buttons as inputs.
Edit:
Another feature request denied by a mod author due to this: http://feedback.rhsmods.org/view.php?id=4572