The Stronghold Crusader 2 Map Editor is located in the Library/Tools menu within Steam.
You must install the Stronghold Crusader 2 game before you install the map editor.
If you experience issues with the map editor not starting correctly, please follow the following troubleshooting steps:
1) Make sure you have installed the prerequistite files required to run Stronghold Crusader 2 Map Editor
Microsoft .NET Framework 4 (Standalone Installer):
C:\Program Files (x86)\Steam\steamapps\common\Stronghold Crusader 2\resources\dotNetFx40\dotNetFx40_Full_x86_x64.exe *
or
http://www.microsoft.com/en-gb/download/details.aspx?id=17718
2) Check your anti-vius software
Some anti-virus software can be overprotective and may prevent Stronghold Crusader 2 from running.
Please either make C:\Program Files (x86)\Steam\steamapps\common\Stronghold Crusader 2\bin\win32_release\MapEditor.exe * an exception in your firewall software or disable your anti-virus software when trying to run Stronghold Crusader 2.
* this is the default Steam installation path for the game