I only see an option to use iChat with your .mac account or an AIM account. Any way to link up with buddies via YIM or GMail chat with iChat? BTW- I have the Leopard OS.
MSN in iChat using a jabber account, take a look here
Just make sure to pick a jabber server that supports the MSN protocol, I use 12jabber. Be aware that you will have only basic chat features for MSN buddies, no Audio nor Video
Personally I like Adium better for all IM services