I think only solid can move a platform sprite, however, you could still move it around with the "Move at angle" action:
Angle : the angle in degrees
Distance : Speed*dt ; where speed is the speed in pixels by seconds
or if you want to move it with X and Y speeds:
Angle : Angle(0, 0, Speed_X, Speed_Y)
Distance : sqrt(Speed_X*Speed_X+Speed_Y*Speed_Y)*dt ; where Speed_X and Speed_Y are the speeds in pixels by seconds