Import this [sky sample file ] to library and create new layer. Pick up the sky to the stage in new layer and convert to symbol. Then click align at left and top edge from Align and Information transform tab. At frame 90 in timeline insert key frame and set align right edge. Back to frame 1 make right click and select create motion tween.
Please remind me at rjairak@gmail.com if you can't view this flash vdo. Thank you.