There was a thread on the FSpassengers.com website that was almost identical to this one and it was determined that it was a case of piracy.

In order to reinstall YOUR key you have to contact Dan who is the creator of FSPassengers, however since you didnt buy the copy of FSP there is nothing you can do except buy a copy for yourself.

EDIT: I have sent Dan a message with a link to this thread. Mods please do not delete or move it until he has a chance to read it.

Thanks