You will need to have the beta version of the client to play today, tomorrow we should have the official update, which is just this beta version converted to release.
In steam go to Library -> Games, then right click Wurm Unlimted and go to properties. Click the beta tab, and switch to the beta branch. Update the beta and you will be able to login to Mythmoor and enjoy the new client. Report any issue with the server so I can look into them