You need to check what version the server is running, 3.1.2 or 3.1.3 . Then, if on windows, type %appdata% in the start menu and go to the roaming folder. Navigate to .techniclauncher and delete the whole tekkit folder. The close and relaunch the technic launcher, press options, manual build and on the drop down choose the right version you need. Press OK then login and it should work :)