Recently i ordered the expansion and today I recieved it but when i try to install it, an error comes up saying the CD Key code is invalid and that its from another product, but i know its most likely not since it says that it is the key on the sticker on the manual. Anyone having or had the same problems or no how to fix this?

I had this problem too, using the downloaded installer. When using the downloaded installer, you need to get the CD-key from your account (on the same page as the download). It's a different key from the one on your case (or at least, mine was).