Mamewah has nothing to do with this setting. You just need to set up one of your buttons in MAME to be your credits button. You can do this by pressing tab in MAME, going into Input's (general), scroll down to the coin 1P and then press the button you want to use for the coin up. it's probably a good idea to set it up to still use the keyboard key 5 as an optional way to coin up for testing purposes and maintenance. To set an or command press the button you want to use as the coin, then wait about 5 seconds and press the other key you wan tto use for coin. It should pop up as an "Button" or "5" Depending on your set up you would also want to set up your coin 2P the same way. except use "6" and your 2P coin button.
Disclaimer: This is just a basic explination. If you're using Joytokey or some other encoding program it may be different for you. You just may want to set it up according to your liking.
If this is confusing I'll try to explain better.
-G