It should be on the plastic case that the disk came in... on a white sticker.

Putting the serial number on the outside of the box would be a bad idea.

problem is, there isn't a white sticker, nor do I have something printed on the box.

what can I do?

a new copy, but no printed manuel. only plastic box with the 2 cd's

Unfortunately if you did not register the game we won't have it on file to help you recover it. If you entered it on install it may be in your windows registry, but beyond that you'd need to purchase a new key.

If you do have the box and all the packaging and there was just never a key there to begin with, contact support@stardock.com and they should be able to help.