I'm sure this will be re-ittirated, but the airplane mode isn't sideloading.
that allows you the ability to access apps ALREADY on the marketplace not allowed by AT&T. You turn the phone off, remove SIM, power on, enable wifi, then download (I believe, I've never done it so someone may have to correct me)
Sideloading is putting apps from third parties that aren't necessarily on the marketplace. Google Sideload Wonder Maching and research that