http://sizeofaex6zgovemvemn2g3jfmgujievmxxxbcgnbrnmgcjcjpiiprqd.onion
Download the last version ( 3.4.23 ) of ICQ available for OS X and PowerPC architecture, and install it. The nina.chat people are nice enough to provide access to an ICQ service that will allow you to use the old clients. Problem is that you need to patch the server address inside your ICQ client, basically, all occurences of .icq.com will need to be changed into .nina.bz .