You cannot login to ICQ through iChat. They do indeed use the same OSCAR protocol, and are in many ways compatible, however, iChat must have been specifically set to disable this (probably because ICQ has different file transfer methods). You can however, get an AIM/.mac account and add ICQ people to your list, as long as they are using an OSCAR ICQ client (simply use their ICQ# as their screen name). Google OSCAR and TOC to get a better understanding.
I would recommend Adium instead, especially over Proteus. Adium is under far more rigourous development, and has a lot more innovations existing/down the pipeline. Not to mention a more stable file transfer and oh yeah.... did I mention it's free? Proteus is shareware (no diss to Justin).