"The product code you've entered is not valid or is not a product code. Please double check to see if you've mistyped your key. I, L, and 1 can look alike, as can V and Y, and 0 and O. If you are redeeming a gift card please click here to redeem it."

"To register your product with Steam and add it to your Steam library, enter the product code distributed with a retail CD/DVD or other Steam product code above. Click 'Continue' to continue.
Keys look like the following:
AAAAA-BBBBB-CCCCC
AAAAA-BBBBB-CCCCC-DDDDD-EEEEE
237ABCDGHJLPRST 23"

When you activate your key, writes this.

Your key looks like: AAAAA-BBBBB-CCCC