How do I get the wheel off

Grandson's '05 basic Mustang with 4.0 V6. Flat front tire so we went to take the wheel( multi spoke alloys) off and fix the flat, lug nuts off and center cap out and the wheel will not come off. I tugged and pulled (she was on a jack so I did not care to pull it off the jack) and wiggled her and still no movement. I noticed in the center hole of the wheel a black plastic cap device with a FORD logo and EP33 markings and wondered if it was some sort of retainer so I called local FORD garage and was told that the car did not come from the factory with a plastic retainer and that sometimes the wheels did stick and to get her on jack stands and then get under and kick the wheel at the 3/6/9 O'clock positions (oh yeah). Bottom line was I pulled a nail out and plugged the tire while it was on the hub. My question is two fold, 1. is there a plastic lock nut or something like that on these wheels (do not know if this was FORD or after market wheel), and 2. How do I get the whell off?

Sorry for the long dissertation, and all responses gladly accepted.