The $35 code you received from gamestop would have covered the cost of the expansion pass. If you would have purchased the pass you would have received both the dark below and the house of wolves. Seeming as you only purchased the dark below for $20 you'll need to to add $5 to your account in order to cover the cost of the house of wolves. You can try calling Microsoft and explaining to them that you intended to buy the pass and not just the dark below.