Hey JLH1964, I just tried it with "Test Mode" = False, and on a device (my Android v4.0.4 / GT=S7560M), and it still won't show up;( Will it simply not shows up on older devices like mine?
Are you sure your ad trigger is working? I'd probably temporarily play a sound when 'counter >= 7', just to be sure your logic is working. If you hear the sound, then you know its your Admob setup that's an issue - not your C2 logic.
In admob, you did choose 'interstitial' when creating an ad ID # - correct?
Also, in XDK, you did insert the admob plugin?