Before you can run the Steam Community Overlay in Battlefield Heroes, which is currently still in beta as of 2-16-09, you will need to install 3 pieces of software:
1. Mozilla Firefox
3. Battlefield Heroes
Click the links above to acquire.
This instructable is courtesy of http://ruckman.net
Step 1: Add non-Steam game
Step 2: Add selected program
Step 3: Go to the properties
Step 5: Change the properties
"C:\Program Files\Mozilla Firefox\firefox.exe" http://play.battlefield-heroes.com
Step 6: Optional step
C:\Program Files\EA Games\Battlefield Heroes\
and choose BFHeroes.exe and press the "open" button.
Step 7: Check the properties
When verified, hit close.
Step 8: Your ready to test
Double click on the Battlefield Heroes shortcut you created in Steam and it should launch your Firefox web browser. It will take you to the Battlefield Heroes website. Log in and hit "Play now".
Step 9: Final results
NOTE: Make sure that Firefox is not already open! Steam must be the program to initially spawn firefox. Otherwise, it will not work and it will only open a new tab in Firefox.