Hi,
Yes iChat 5 (in OS X 10.6.x) has no Yahoo Option.
iChat 6 from Lion will not work in earlier OSes.
Each version is bound to the OS X level that it should be with.
This applies to sub version within the OS X update life cycle
Essentially iChat 5.0.3 will only work in OS X 10.6.8
The previous public version (version 5.0.1) needed OS X 10.6.5 if I remember correctly.
iChat 5.x.x should be part of the Install for Snow Leopard
Check whether it is still in the Applications folder (removing it from thre DOCK does not delete the app)
If you do remember deleting it from the Applications folder you can reinstall it from the Install DVD
You would then need to update it to the latest version fo the OS you are using.
To do this you need to Download and install the COMBO version of the last update.
This will update iChat form the version on the DVD (Possibly ichat 5.0.0 in effect) to the iChat 5.0.3 version.
You would then need to register an AIM name (I would tend to shy away from the option to make an existing email your actual online Name)
Functions of iChat.
The AIM Name would then be entered in to iChat and the account/Screen Name would login.
(there is an "Enable this Account" box that can be unticked to Log out - you can also use the iChat menu > Accounts and Untick the account there)
This login is by default on port 5190 using the TCP protocol.
This accesses your Buddy list.
You can then add Buddies.
Buddies that have Cameras and Microphones end up displaying green Video and Audio Only icons (if no camera).
These iCons or highlighting a Buddy and using the ones at the bottom of the Buddy list can start a Chat (invite).
The Video (or Audio Chat) can be Peer-to-Peer by the time it has finished connecting.
In iChat 5 in the iChat menu > Preferences > Accounts > Security tab you can elect to relay through the AIM servers. (it's difficult to tell if this actually helps)
Messaging
Depends what you mean by Messaging.
iChat is an Instant Messenger app.
It can join the AIM Service or use Jabber IDs and send text messages (IMs) to Buddies as well as Audio or Video chats.
It can also do Screen Sharing.
IMs (text Chatting) will do so to many text based apps that can login in the AIM service or do versions of A/V chats that are not compatible with iChat.
AIM do a version or at least allow a Login via a Web Browser called AIM Express which can be used for Text Chat only.
In the United States you can SMS from iChat 5 to certain US based Cell/mobile phones if they are on the "right" carriers.
Technically the computer has to be set up as if it is in the United States.
The Phone number is then added as if it were a Buddy as in +1234567890 (where the +1 is the country code)
(as it was never rolled out world wide I am not sure why they insist on the Country Code)
As I have stated the phone HAS to be in the United States on a carrier that supports SMS forwarding from AIM.
It is true that Messages in Mountain Lion adds Apple's own iMessages service which is text Messaging to iOS devices as well as the Message App. (Messages is iChat+ with the plus bit being the iMessage service)
It still does the SMS to AIM "Buddies" but not via the iMessages service.
The iPhones can use the iOS version of Messages and it will send SMS to Non iMessages numbers.
To Get Messages you would have to Upgrade to Mountain Lion as Lion does not have Messages (it has iChat 6 which would give you Yahoo - but that still does not do Video Chat in iChat)
10:03 PM Tuesday; December 4, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
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