The trick to the Nav+ is that it hides the "Scan" button that the other Audi headunits have. To get the equivalent function--for instance, to set the "Randomize all" function, you have to select the "CD List"(upper left) option. This will bring up the 6-CD list. scroll down and select "CD 6". You can now use the "Track left" button above the knob until you hear "Randomize all". At that point, you have to select "CD Control" (right bottom) and turn the knob until it's on "Scan" and press.
Selecting CDs is easy if you generate one playlist per CD. You can then use the playlist select (CD 2 option) and use the same process to select them.