I have to warn you that Yahoo Messenger for OS X is not up to par with Windows client. But it is the only viable and free solution for OS X ... You may want to try Charla 2.0 (OS X only) client for Yahoo. It supports video/audio just like Yahoo's own messenger and has some important advantages as well.
One this I found to be completely missing from Mac side of instant messagin was the ability to efforlesly voice/video chat through MSN/Y!/AOL messenger clients on OS X ... and thats besides the fact the OS X client are utter crap for lack of a better word --- I wish all the beforementioned companies would get their **** together and make a descent client for OS X.