Although, you’d expect iChat AV to function without much set up, sadly, there are a few ports that must be opened before iChat AV can function. With the advent of Mac systems with built-in iSights these days, this issue is becoming especially cogent.
By default, Mac OS X installs with the firewall switched off, so unless you’ve intentionally switched it on under the Sharing pane of your System Preferences, iChat AV should function fine. That is, unless you’re behind the firewall your network has built in somewhere else.
When using the built-in Mac OS X firewall, you only need to open these ports: 5060, 5190, 5297, 5298, 5678, 16384 through 16403. Or if you’re behind a network firewall, then you must open the above ports on your network firewall. If you’re not sure what we’re going on about at all, its best you consult your network administrator or falling short of this, hire some professional help.