Cain + Abel will only find the aim pw if it is saved on the computer (remember password, etc). If you can sniff the md5 hash that it sends (if it still uses md5) and then you could try looking it up through rainbow tables.
Also, cracking wont work very well through AIM express, oscar + toc both rate you after several tries. The only method would be a multi socketed cracker which ran through a fresh proxy after several attempts. This method would also be pretty slow.
