You can also try Colloquy.
http://colloquy.info/ (Download it from
http://colloquy.info/downloads/colloquy-latest.zip )
Other option is Adium (
http://www.adiumx.com/ ) a multi-protocol client based on Pidgin. Server I use for Pidgin is: paix.irc-hispano.org
Alternatively you can try and fix Java problems if you want to use the Web/Browser based Chat link. You can try to see if your java works by going to this link
http://www.javatester.org/version.html. If you get the pink box with Java version, you should have Java installed and working, however you might need to update to a newer version. I don't know how update process works on Macs to give you advice.
Your problems also could be due to internet security software, popup blockers or anti-ad programs too.
I haven't tried any of this software on a mac as I don't have a Mac. As always usual precautions apply (ie Virus Scan, don't install if you or your antivirus/security software think its a virus/malware/spyware).
-SS