When you opt in for the BETA, ForgeEdit will be installed. To run the editor you will need to browse the directory where D20 Dungeons is installed. You can create a shortcut to ForgeEditSteam_Win32.exe in the last step to save you from finding it each time.
Here are the detailed instructions:
1) Open Steam.
2) Find D20 Dungeons in your Steam library.
3) Right click on D20 Dungeons to get the popup.
4) Click on LOCAL FILES.
5) Click on Browse…
6) This will bring up the folder where everything is installed. Open the tools folder.
7) Open the ForgeEdit folder.
8) Double click ForgeEditSteam_Win32.exe