In the button definition, just use the name of the function in the "credits.c" as if it were in the same script. Seperating scripts doesn't do anything apart from help you organise your code. They all get merged together when the game is ran as if it was all one big script.

So if in your "credits.c" script you have a function called StartCredits(), then just use "StartCredits", in your button definition and the engine will automatically find the function in the other script and run it.