I am posting this on behalf of someone trying to join my server.
We are using literally the same exact minecraft.jar file. They can start up single-player just fine and play. They always get this message when they try to join my server (which I am currently on as I write this):
Disconnected by Server
This server requires you to have Minecraft Forge installed.
I am running the server (just the two of us) from my Mac and they are trying to join in Windows, could this create problems?
We are running all the latest builds of Redpower 2, Buildcraft, and Industrialcraft 2, as well as Forge and Modloader.