I just found out the answer for myself, by a stroke of luck. Good thing that I left the Bruckenpionieres close to the Transport Airfields...and that I saved my turns!!!
The 4 Bruckenpioniere Units that were landed off the Ju52's must occupy the Transport Airfields at the same time to end the scenario with a Decisive Victory.
What I was doing was landing one, then sending the Bridging units out to battle, figuring that the computer was keeping a tally that I had landed the first unit, etc. Not the case. Glad I could salvage a Decisive Victory, though. I was not too keen on replaying the whole thing so soon after I wiped Ivan clean off the map.