C:\Program Files\Steam\SteamApps\common\Football Manager 2011\tools\editor or
C:\Program Files (x86)\Steam\SteamApps\common\Football Manager 2011\tools\editor
If you are on a MAC
Finder > Macintosh HD > Applications > Sports Interactive > Football Manager 2011 > Tools > editor > Editor.app or
/Users/*YOURUSERNAME*/Library/Application Support/Steam/SteamApps/common/football manager 2010/tools/
I can only confirm the windows steam method 100% as thats how i have installed the game, but I'm 99.9% the other installation methods won't have changed the location of the editor from FM 2010.
If anyone has installed on a mac or on windows without steam and has found the locations i have listed to be wrong, please PM me.