"(1)" represents the main button you'd use in MAME. Such as when you move through the emulator's menu and make selections.
You can also look in the Control Panel in the XP startup menu. See what buttons are listed on the screen and how they correspond to the actual (real) layout on your control panel by pressing each one.
I will mention, every time I start with a clear (no previous save(?) file) for a game such as a fighter by Capcom - it seems like the top 3 buttons are the kick and the bottom 3 are punch. I always have to reconfigure these in the MAME set-up menu.
eta: Needless to say, I didn't cover anything of what the 2nd player side would be. I've never used... I mean. I ain't go no friends to pway wif.