Hi,
Whilst it is true the @mac.com option no longer gives you access to the Registration Page you will also find more recent Changes to AIM acceptance of @mac.com and @Me.com ID is also limited.
At the end of June 2014 the single Login to AIM with an Apple (issued) ID stopped working.
Later versions of iChat 6 and the messages app in Mountain Lion and later do a double login to both Apple and AIM that allows one to verify the other is allowed to read the password.
I.e. the process for logging on used in Leopard and iChat 4.x.x is not deemed to be secure enough. The same is true of your earlier version as well.
I have a G4 that is running Leopard (OS X 10.5.8 to be precise and iChat 4.0.9) so you could update one more OS (I am not sure whether it would cost or not and there may be little benefit from doing so as far as iChat goes)
You should also have the Jabber option.
This can be made to work with Google and Facebook if you have those IDs.
As I take it most of your Buddies are AIM ones your only real option is to register and AIM at AIM and use that.
AOL is AIM. Start here https://settings.aim.com/ and th get a Screen Name option top right.
10:08 pm Monday; January 19, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad