Counter not working (Shift Key)

[K_is_king]

So basically I game on a 27" Mac. Pretty pathetic, but it can run every game 1080p medium settings quite easily. The thing is, my mouse is an Apple Magic Mouse so when I right click to aim, I can't fire the gadget (left click). So I mapped Aim to Left shift. However, it says that the button for counter is also shift and it doesn't specify which one. I'm assuming it's right shift but during gameplay I've tried both the shifts to counter (in the first scene with Catwoman) and none of them work. Anybody else having/had this problem and knows a way to fix it? Thanks in advance.