The trick with MP is that you must have EXACTLY the same files as the other players/host.
For example, a few of the GWX Beta Testers get together quite regulary to play MP. The only extra file that we have enabled is the Dec 23rd Fix.
We also have 2 installs, 1 for our normal career game (modded to suit) and 1 totally 'untouched' clean version for MP.
In your instance I have to assume that you are falling foul to problem of not having the same files as everyone else.
Edit - Do you know these people that you are trying to join on MP? If not then you might never be able to join in with them as they may have a predecided set of mods enabled. We (SubSim Members) have set the standard rule that the above setup is the only one used so that people like yourself can easily join if you want to.
Also have a read of this thread -
http://www.subsim.com/radioroom/showthread.php?t=102738