HI,
Just to clear some description type things up.
iCloud is essentially a service that Apple Provide.
The ID you register with it can be an Apple ID that you have had for some while and may be an external to Apple email (mine is from a Dial Up ISP)
The registration process gave me an @me.com name as I did it close to the end of MobileMe and the start of iCloud.
I understand now you get an @icloud.com email.
So you may have an @iCloud Email as you have registered for iCloud.
You may alao have an Original Apple ID (which is an email) that is now linked to the iCloud Service and deemed to be your "iCloud" ID.
Either the original ID or the @iCloud.com ID gets you access to iTunes Store, App Store, the iCloud web page, the iCloud Services from the System Preferences such as Mail and Contacts syncing etc - whilst using the same password.
I am not sure if you can register for iCloud without providing a Recovery/Verification email as I converted an existing Apple ID into an iCloud one.
Many people who were using MobileMe before hand also "Converted" in this way.
If you only have an @icloud.com ID then enter it as if it were and AIM name but type it in full (username@icloud.com).
In Messages in Mountain Lion this becomes the standard way to enter all AIM and AIM Valid ID.
If you are trying an AIM registered name then it should work if you enter it the same way as the name is entered elsewhere. Commonly this is the username only with no suffix (@aol.com or @aim.com).
Port 443 is normally open in the Mac Firewall when it is on.
However the Mac Firewall also needs Apps to be listed to be allowed when it is On.
Port 443 should also be open in your router.
I delayed getting a Lion capable computer (I have an early intel MacBook Pro and thought this would cope).
I then switched to Mountain Lion without preserving an Lion partition anywhere.
As consequence and due to time restraints at the present time I have not updated my website.
It has less on iChat 5 than it does on iChat 4 but there were very few changes.
iChat 6 only really adds the Yahoo option to a fairly stable app.
The main changes are in the support structure.
IMAgent takes over for iChatAgent and it also deals with FaceTime.
In /Library/Frameworks and Private Frameworks the items that "run" the app are in different places and involve different items to earlier versions.

9:27 PM Wednesday; May 8, 2013
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.3)
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
"Limit the Logs to the Bits above Binary Images." No, Seriously