Make sure it's a wired controller and not a wireless.
Also, I would try and learn the keyboard and mouse as opposed to the controller. The keyboard can give you easy access to powers and what not of both you and your teammates. It's more fluid than the controller.
Gamertag: KumfyKittyKlub "So long as there are men will there be wars."
Mass Effect 3 does not support native Xbox 360 controller support for the PC as the UI is completely different. You may emulate it manually but it won't work as well as it is designed for using a mouse and keyboard.
It doesn't but there's a way to get it to work, i think it was an ini edit. Works with ME1 too, I believe.
I use my wired 360 controller with the PC version I got from DA2, and the only thing wrong with it is there is no cursor on the powers wheel/main menu, and you have to move around until what you want is highlighted.