'Game not Found' on multiplayer games

[RabbitMan]

My girlfriend and I are trying to play this, we have two different copies from steam, no firewall issues, and whether we are on LAN or metaserver or even connecting from IP, we can neither see nor connect(in the case of Meta). Please tell me there is a simple fix.

 

[AndrewT]

Are your checksums a/ correct and b/ the same as each other? You can see the checksum on the main game menu, or when you go to host an MP game.