The Dealer is probably clueless because the crossbars are usually installed at the Port and not the Dealer.
By the way, did you get the bag with the wrench and rail covers? That bag more important than the "key". The key is just a plastic knob that is used to turn unscrew the end covers on the crossbars. If you ever want to to take the crossbars off, you'll need the wrench and the decorative covers that cover the empty spaces left by the crossbars. (My crossbar "keys" were actually in the bag with the other parts and the instruction paper.)
Also, the crossbars aren't "adjustable" like you suggested in your original post, meaning they only install in one single location and can't be moved forward or backward.