Check out this post: http://www.clubtouareg.com/forums/f43/diffential-lock-rotary-knob-question-18900.html#post125826
As shown in the picture, you need to have the icon in the 6 o'clock position of your High - Low - Center Lock - Rear Lock selector knob.
Edit: The icon in the 6 o'clock position is your rear diff lock. If you don't have that, you don't have a rear diff lock.