THIS software then:
http://www.ultimarc.com/winipac.html ?
The following assumes you wired all inputs to defaults.
Looking at the screenshot, with 1-2 Normal Keys selected you should see the little white SHIFT icon next to the 1P Start button. If it isn't, click the 1P Start button on the screen and then click the "Set Shift Key" button at the right next to Player 1.
Next, select the 1-2 Shifted Keys tab and make sure Enter is displayed next to the P1 Joystick left arrow. If not, click to highlight the input, and then press Enter to change it to Enter.
Then click "Program I-PAC" in the lower right.
If this fails, try hooking up the I-PAC temporarily in PS/2 mode, and make sure the options tab is configured for the I-PAC/2 and not the I-PAC/4
HTH