What I want to do is put Google Ads in my flash game.
The first question is how can I do that?
The second question is the best place to put it; while loading, after finishing the game?
Last I checked you cannot embed Google Adsense in a Flash game/app, because Adsense is HTML+JS, which cannot be put inside Flash.
You could hover it over your Flash game...but there are compatibility issues, doesn't work on all OS/Browsers, and may cause UI bugs with your Flash game.
You could try hacking the Adsense code so that it requests the ad data and loads it & displays it inside Flash...but this is against their terms of service.
Your only option with Adsense is to place them on the same page next to your flash game (which isn't an option if you need your game to take up the full page). I suggest looking at Mochi ads, they are specifically designed to be embedded in flash games.
Edit: A user posted a link to Google Adsense for Flash Games...unfortunately they have quite strict requirements, which most people wont meet.