Yeah this happened to me, I went back to EB today and they gave me the piece of paper for the stealth cars. It had individual codes for each car, unlike the chromeline that was one for all of them. Just go there and tell them they never gave you one. I had to make sure they understood what cars I meant because the initial guy I spoke to said the only cars you get are the ones for collectors edition called chromeline (he was clueless about stealth pack)
But yeah, it is there, most likely on the main counter, you just have to ask the right person.
Havn't used any stealth cars yet, but I gave the chromeline M3 to my B-spec driver to drive and so far he's been winning them all.