I want to use a gamepad while playing the game(I don't like mouse controls) but when I tried re assigning the controls in the options menu, the game wasn't recognizing gamepad button presses.

I can use the controller in other apps on my PC, so I know it's connected.

Does it require some sort of calibration, or an add on? Or does the PC version not support controllers? (Odd considering the recently announced port to consoles)

The Enhanced Edition will support controllers on PC, but the current release does not (it is still in the process of being added, and the UI finalized).