If the firewall is off, you don't need to do anything.
If you need to make changes on this screen, you will need to click the lock icon (bottom left) if
it's closed.
In Snow Leopard, click the Advanced button
If Allow all incoming connections is selected:
Presence works with this setting. You have nothing more to do.
If Allow only essential services is selected:
This setting stops Presence working. Select one of the other two
settings.
Open Presence and click Restart.
If Set access for specific services and applications is selected:
If there is an entry for PresenceDaemon in the white box, and it says Block incoming
connections, change it to Allow incoming connections
If Presence is not working, and there is an entry for PresenceDaemon in the white box,
you can try removing it: select the entry, then click the minus-sign button.
If there is no entry for PresenceDaemon, or if you just removed it:
Open Presence and click Restart
You should be asked whether to allow Presence to receive incoming connections. Click
Allow.
You might need to click Restart again after doing this.