Sorry, I'm not really sure I can help you with this one. You might want to try the steam forums.
Some people said to launch the SDK with "-engine ep1" in the launch options, but I think this was for a different problem. You could give it a try I guess.
im going to take a shot in the bucket and guess the solution, i think (becasue i had a similar problem before) you should go to programfiles/steam/steamapps/name/zombiepanic and double click on the zombie panic source EXE file.