There are a couple of ways to do it, the easiest is within steam.
Step 1: Right click on "Ashes of the Singularity", and click "Properties"

Step 2: In the "General" tab, click "Set Launch Options".

Step 3: Type in "-modding", and click OK, then close the properties window and you are done.

Then when you run AotS you should have a Map Editor button in the bottom left hand corner.
