Get the NAV system going, including the "I Agree" stuff. Press the Menu button (top right), (if you are moving the Map DVD button on the MFD will not be enabled) and then touch the Volume Icon. On the following screen, tap the upper left corner (just inside the frame), followed by the lower left corner, then upper left, then lower left. Top, bottom, top, bottom. After those 4 taps, a menu should appear that includes OVERRIDE. Press OVERRIDE (DO NOT TOUCH THE OTHERS: SOME ARE DANGEROUS) and hold it until you get a confirming beep, and then release. Then use the Back buttons until you get back to the Menu screen. If everything went OK and you are moving, you will note that Map DVD is enabled. Try again if not.
With practice, you can do this while driving, but be darn careful. Best way is to engage it quickly while stopped...