Problems with Modding a Multiplayer Server

[Sparrow]

So my sister and I have been playing on a server and I wanted to add Hydrocraft to our server. I host the server, so I downloaded the latest version, added the line into the server.ini and started the server. I made sure she also had the latest version installed. I can log into the server and play and the mod works.

 

But when she logs in, she gets an error message that her game files don't match the server files.

 

I've looked all over but I can't find simple instructions to fix this problem. A simple step-by-step how to might help, or if anyone knows something I may have missed, please let me know.