We are running the Beta 4b Server... which means you must be running Hidden:Source Beta 4b. Get it now from www.hidden-source.com.
Even after you install Hidden Beta4b, you may need to do a couple things to play on our server.
Newer versions of Hidden/Steam have turned off the automatic ability to download custom maps in Hidden. Since the WallWorm server runs custom maps, you will have to turn this feature on if you want to play without getting kicked on change to a new map. The temporary solution is to go into the Console and type cl_allowdownload 1. That will allow Steam to download maps while you are connected. A more permanent solution is to edit your userconfig file to include that line.