Messenger - request remote assistance - McAfee firewall settings

Live Messenger Messenger 8.5
Requesting PC - WinXP SP3
Assisting PC - Vista Ultimate SP1

How do you configure McAfee firewall to permit port connection on request
for assistance. My Windows firewall automaticall ydoes this but I cannot
assist my brother with McAfee protection from camcast. He sends the request,
I accept and then it times out with a message on my assisting PC that the
person on the remote side is not answereing.

any KB help

Thx

John L