From Google:
Requirements
If you want to integrate AdSense for games into your Flash-based games, you’ll need to make sure that you adhere to the following prerequisites:
Ensure you're compliant with the AdSense program policies, including these specific games policies.
Utilize a game launcher capable of Google IMA3 SDK integration and/or compliancy with VAST 2.0 / VPAID 1.0.
Utilize a Flash-based AS3 game launcher: AdSense for games is only available for Flash-based game content.
Have a high volume of games content, i.e., greater than 70% games content with over 2 million games impressions monthly.