Trying to Install Photoshop CS6 on PC

Hey there, I'm trying to install Photoshop CS6 on my PC. I originally bought it for my Mac laptop and desktop, and now would like to move on over to my more powerful setup, but... alas, I cannot get the product code to work? Is this because I originally ordered a CD for the Mac and not PC? I don't have a disc drive on my PC, so I downloaded Photoshop CS6 directly from this site, so that's no issue. It's just that it won't take the product code on the box I have here. Am I out of luck? (Also worth noting, I had it up and running just fine on my other two setups before, but for whatever reason I guess I never got it linked to my Adobe account, so I tried to punch my code in there too but got a 413 warning, "header is too large".)

Anyway, I'm so confused!

Thanks so much in advance.