What operating system are you using?
Which version of camera raw do you have in photoshop cs6?
After you select the Adjustment Brush, the New button should already be enabled.
Then after you draw on an area of the image, the Add button should be enabled, with the option to choose either of the other two buttons; New or Erase
Could you post a screenshot?
Maybe all the adjustment values are set to zero?
(at least one needs to have a value other than zero)