In DAVID 3.8 (and newer versions) the setting of several MMM doesn't work like in DAVID 2.4.
The motor speed can be chosen in the '3D Laser Scanning window':
Goto 'Motor Scanning', Set a hook at 'Record New Motion', choose the right 'Motor Speed'.
'Start' the reference scan. After this scan the object scan can be made. Of course with the same speed. Therefore 'Motor Speed' is now gray displayed, means not modifiable!
Before this works you have to set the right COM port address (in DAVID's Hardware Setup) and you must set a hook in the Advanced Settings: Communication, COM, Enabled.