MobilePlatformVelocityMotion is an internally used object. What you are trying to accomplish would require a MobilePlatformRelativeMotion parameterized with a setVelocity command instead. Syntax would be as follows.
MobilePlatformRelativeMotion test = new MobilePlatformRelativeMotion( dx, dy, dtheta );
test.setVelocity( transVel, rotVel );
units are mm and rads.