From the MS website :

"To enable voice and video communications with Windows Messenger through a non–UPnP firewall, configure the firewall to allow incoming traffic on UDP ports 5004 – 65535. (are they kidding here?)
For other purposes, enable the following ports:
File Transfer: 6891 (to allow 10 simultaneous file transfers open ports 6891 through 6900)
Application and Whiteboard Sharing: 1503
Remote Assistance: 3389"

I've tried opening port 6891, but that doesn't seem to work (or I'm doing it wrong)
I need to enter a "trigger" (whatever that may be), and the port value. But everytime I click "save", the port value dissapears.
I don't know what I'm doing wrong.