let me know if this works -

Press and hold the “TURBO” button. Once the “TURBO” button has been depressed, you will have a five- (5) second window to begin the programming process below:

1. While continuing to hold down the “TURBO” button, press the action button you want to assign turbo to: a. One (1) time for fast turbo speed — the turbo LED will flash rapidly. b. Two (2) times for slow turbo speed — the turbo LED will flash slowly. c. Three (3) times to cancel turbo — the turbo LED will go out.
2. To assign turbo to other action buttons, repeat step one (1) above.

+

fast forward to 2:47

should be the same idea.