Noticed your using the Mac version...
(1) Has this problem only just appeared now, or is this your first time using the Level Editor?
(1a) You have a ".map" file somewhere on your HDD, right?
(1b) Can you browse to it (Don't look at it through the "File>Open" menu in the LevelEditor) and make sure the file has a ".map" extension.
(2)
http://wiki.frictionalgames.com/hpl2/tools/start
In the Mac OS X part, have you gone through everything relating to the Mac Version? Their might be bugs or what not, I haven't myself tried installing the tools on a Mac yet.