Rebuffing Reduction (Combat):
"Your damage reduction can turn the force of blows back on your enemies.
Prerequisites: Str 13, damage reduction, Improved Bull Rush, Power Attack, base attack bonus +1.
Benefit: Whenever an opponent that is adjacent to you fails to penetrate your DR with a melee attack, you can spend an immediate action to attempt a bull rush combat maneuver against that opponent. If you succeed, you cannot move with the opponent.
Normal: A bull rush combat maneuver is a standard action."
My question: does this mean that any time an adjacent character melee attacks me and MISSES, I can spend my immediate action to attempt a bull rush? Or only if it HITS but doesn't penetrate my DR? A failed attack doesn't penetrate my DR either.
No, a miss doesn't cause the free bull rush, only when your opponent fails to penetrate your DR with a melee attack. DR only comes into play once you're been HIT, and damage is being rolled.