As I said, it has to be one of the GAME buttons that are defined. Unless you are using a custom input script, you will be limited to the following:
UP, DOWN, LEFT, RIGHT, F5, F6, F7, F8, F9, CTRL, ALT, SHIFT, L, R, C, Z, B, Y, A, and X
In the config, each of these must be preceded by Input::
Now, keep in mind that C does not mean the C key on your keyboard, but the C button defined by pressing F1 during gameplay, which is by default the space bar and enter key. The same goes with the other single letter keys.