In iChat 4 and iChat 5 if you selected either MobileMe or @mac.com and then pressed the Get an iChat Account you used to get taken to the @mac.com registration page
As you say, it now redirects you to that Apple Doc about it not working.
I am not sure why they have done that as the iCloud Issued @me.com names will only work in iChat 6 and the Messages Beta.
I have linked the the @mac.com registration page.
You will need another email (it links one with the other).
This email cannot be one that has already been used to create an Apple ID.
In addition the password must be no more than 16 characters if it is gong to be used in iChat (AIM Server Limit)
If you have forgotten a Password for @mac.com then you can go here at iForgot
It can be changed On line if you no longer have access to the email (but only if you remember the Security Questions and Answers)
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.3)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously