that message means that the host is using a different modlist than you are, they have to match. Hope this helps. If you are using the same modlist then I'm not sure how to fix it.

you can do multiplayer. the people that you play with should have the same mods installed and in the same order. (Example: if Improved Space Battles is the top of the list as 1 and if someone else has it as 5 it would not work. its very stupid)