http://i748.photobucket.com/albums/x...72abf24053.jpg
http://i748.photobucket.com/albums/x...11d30e0e44.jpg
The problem is, you have auto leveling on so you can't select where you want your power points to go. Respec, then put one power point on each skill in the MIDDLE row. Then you should be able to use Dark Binding.
Oh right! I figured. Shao's right. You cannot unlock and rank dark binding because you have no more skill points available to distribute. :/ Follow Shao's advice, though. After re-spec-ing, you may also want to consider putting skill points for passive powers, the ones shown in the row below the skill tree. This will increase either attack, defense, health or damage. :)