Don't feel bad...I looked in the manual and STILL couldn't figure it out.
So I played with the black plastic thingy around the dipstick until I found that it unclipped from the top and hinged at the bottom, thus, unlocking the dipstick.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car