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.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car