Not sure what to suggest except go back to Tech Support and have them correct it.
Unless entering the IP of the other machine under SecurityCenter > Web and Email Protection > Firewall > My Network Connections perhaps?
To trust a computer the version of mcafee installed on both computers must be sameeg: WSS12.8 and the network type should be same on both computers eg : home, work or public. Adding the IP address to SecurityCenter > Web and Email Protection > Firewall > My Network Connections should make sure the computer shows up in home networks in McAfee.
If you are unsure of the IP address of the computer, just type <ipconfig /all> in cmd and look for ipv4