Getting mouse button to work

I just got a new mouse that has an extra button where your thumb goes. I have tried mapping it in the game and it saves fine but in the game it doesn't work. I have also mapped it in windows to a keystroke and still doesn't work. Would this be related to the same status that gamepads and joysticks are not supported yet or am I missing something? I was trying to set it for for last weapon (Q).

Thanks.