Did you install the latest LeakNet build? 5342
Also, you can try to exit Hammer, remove bin\GameCfg.ini and HKEY_CURRENT_USER\SOFTWARE\Valve\Hammer registry path, and then try to run and configure Hammer again with a special .bat-file:
set VPROJECT=..\hl2
start wc.exePlace it in your "bin" folder of the game
Make sure you have done properly all the things described in
this articleAnd if this will not help -
read this post