Hi,
The Mac Firewall is in System Preferences > Security > Firewall Tab
If On it will tell you (It an Off or ON setting).
If On - then click the Advanced Button.
Stealth must be Off for iChat and you will need to Enabled "Allow Signed Apps" and iChat should be in the list
Apple allow IDs to have password that have more than 16 characters.
However the IM Servers cannot cope with more than 16 characters.
You may need to change to @Me.com password
Most routing devices have the first 1024 ports open by default.
iChat uses ports above this and they need to be specifically opened.
Skype has "Fall back" ports if you do not open the port listed in it's Preferences.
It drops back to either port 110 or port 80 (port 80 is used for Web Browsing and 110 for Mail)
This means that iChat gets through your Modem without you doing anything and in fact working around anything you may have set up. (easier to set up but less secure)
After the Mac Firewall has been Checked then iChat has to be allowed though any Routing device as the ports , particularly for A/V chats, are unlikely to be open.
Use UPnP (Universal Plug and Play) if you device has it.
8:25 PM Tuesday; September 7, 2010
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
