Hmm well, if you're sure you want to do this...
Right Click on the Docklet, select configure.
Select the meters tab.
This particular docklet has a pantload of meters, but they're well labeled, so select the "X free" meter in the box on the right (where X is the drive you want to change).
Click Configure on the left.
In the Scale drop down, select 1M (for 1 Mega...) and click OK.
This will give you the right number, but it will still say "GB" on the meter.
You can go to the Overlays tab and select the "free X" overlay.
Click configure.
In the Format field replace GB with MB and click OK.