The way we used to redeem Steam keys via the Humble Bundle was nice and simple, you'd click a button and it would automagically add the game to your account. This is all changing now however as Steam have decided to no longer support OAuth, which was used to login to other websites via your Steam credentials.
Humble Bundle announced today that they will be going back to the old system which required you to manually copy and paste your Steam key into the "Activate a product" section in your Steam games library. The change affects any purchases that were made after January 12th.
There has been no word from Valve as to why they are going to be making the change. OAuth is something a lot of websites used to verify accounts that own the game on Steam and more. Once hunch from VG24/7 is that they are doing this in a bid to cut down on the number of users who have been infected with Malware.