Made builds with 0.10.0-alpha, 0.9.2a and 0.9.2 versions of npinochet's repo. I'm getting the same error message...Ok, so the problem is that you're using the normal/vanilla version of LÖVE by using Martin's repo, you have to use mine (npinochet's) to use the AdMobs functions. So just rebuild your game with npinochet's repo and it should work.
One funny thing is that with this repository, the game accepts touchs to jump the "intro" to the game, but once it gets to the main menu, the touches stops working (all buttons became useless, but in PC they work normal)
I'm almost giving up, AdMobs don't even return that much money. The thing is that my boss asked me to do this task, and I wanted to deliver, but I thinks this is beyond my programming skills, it juts won't work! But I appreciate all the help you guys gave me.