I've had this problem, and it was actually a problem with MAME and not the ipac (I don't think). It seems to happen after you pause the game and then try to configure. Enter shows up as "Enter Keypad Enter", Shift shows up as "Left Shift Right Shift", etc. It happens quite often when pausing/configing in MAME so I think it's a bug.
The way I get around this is by starting up a game and just configuring the controls without pausing. Once this problem starts happening, it won't go away until you quit out and restart the game.
- Mike