From this Post : put in onstart method.
LinearLayout.LayoutParams adParams = new LinearLayout.LayoutParams(
getWindowManager().getDefaultDisplay().getWidth(),
getWindowManager().getDefaultDisplay().getHeight()+getWindowManager().getDefaultDisplay().getHeight()-50);
adView = new AdView(SimpleGame.this, AdSize.BANNER, "your Ad ID");
adView.setAdListener(SimpleGame.this);
AdRequest request = new AdRequest();
request.addTestDevice(AdRequest.TEST_EMULATOR);
adView.loadAd(request);
CCDirector.sharedDirector().getActivity().addContentView(adView,adParams);