Cannot activate Demigod

Hello,

I recently bought demigod from Stardock's store, which then emailed me a steam key. I got the game onto steam, but after pressing play it tells me to type in my stardock username and password and paste a different code (CD key from steam) into the associated fields. This gives me an error message telling me that this product has already been registered to a different account. I tried activating the product on stardock's website with the code from stream, but that gave me the same error. 

any help would be appreciated.