If you bought it then surely the key would come with it. Try getting in touch with the manufacturers and if you can convince them that you don't have an illegal copy then they will give you one.
if you purchased it and not downloaded a copy the cd key should be on the case on a sticker under where the cd is, but if you bought it second hand then the original owner could have removed it before selling it the shop to ensure that he will never be barred from the online game (as to play the game you dont need to insert the disk) if this is the case take it back to the shop as it is next to useless.