Bsoder,
Launch MAME, run a game. Press TAB, Go to Player 2 controls, press Enter. Look at the controls. Highlight a control, Press ENTER, Then press a button on your IPAC, (it will remap that key for you)
Repeat this step for all your controls.
Here are the defualts, Yes by default player 2 ONLY HAS FOUR BUTTONS (YOU WILL HAVE TO REMAP ANYWAYS FOR GAMES LIKE STREETFIGHTER)
Please do yourself a favor and simply remap MAME to what your IPAC is currenly configured for.
Here is the info you requested anyways. (not recommended)
Player 1:
Button 1 - Left Control
Button 2 - Left Alt
Button 3 - Spacebar
Button 4 - Left Shift
Button 5 - Z
Button 6 - X
Button 7 - C
Button 8 - V
Button 9 - B
Button 10 - N
Start - 1
Coin - 5
Up - Up Arrow
Down - Down Arrow
Left - Left Arrow
Right - Right Arrow
Player 2:
Button 1 - A
Button 2 - S
Button 3 - Q
Button 4 - W
Up - R
Down - F
Left - D
Right - G
Start - 2
Coin - 6
Player 3:
Button 1 - Right Control
Button 2 - Right Shift
Button 3 - Enter
Up - I
Down - K
Left - J
Right - L
Start - 3
Coin - 7
Player 4:
Button 1 - 0 Pad
Button 2 - Del Pad
Button 3 - Enter Pad
Up - 8 Pad
Down - 2 Pad
Left - 4 Pad
Right - 6 Pad
Start - 4
Coin - 8