I use pre-roll ads from 'Ad4game' by putting their code into the index.html file (before the ending 'body' tag). I guess that much of prerolls of other networks work that way. So, in this case - no need to to anything specific within the C2 project, you just put the code in the index after you compile the game.
However, in some cases I use iframe plugin (HTML_iFrame_Pode) to show some static ads during the game. In that case, you place a plugin instance on the desired layout and set the url of the ad that you want to iframe. Then, I add some custom 'close' button to allow players to close the ad.