To connect pots (100 kOhm) to your OI, you'll need a DB15 gameport connector. You can cannibalize a bad joystick for one, get them from whereever you got the connector for your competition port dongle (they're the same connector) or find a rather obscure electronics store that still sells them. (I found one just by asking around at the big electronics stores, and someone mentioned it. Cable and Connections on Leaphart Road in (West?) Columbia SC, if you're interested.) Make sure you also get a backshell, to keep from accidentally grounding pins you didn't want grounded.
From there, follow the wiring diagram in the OI manual on
www.innovationfirst.com and check it before you plug things up.
Hope this helps!