I got caught the same way and enquired by an alternative means to this forum.
It is not a registration key for the running of x-plane. It is a 'market" ploy key for the seller of the software which, if like me, is aerosoft. There is no connection between both parties other than aerosoft being an official or authorised reseller of the software for Europe and the Pacific Region on behalf of Laminar Research.
I have been advised by Laminar Research there is no registration key for the DVD version unlike the digital download version. You will have to insert disc 1 into your DVD drive every time you run X-Plane. A pain in the backside. Unlike version 10 there is no home use USB dongle available. When it will be available, if at all, who knows? A sideways step at present.