I've made a little example ->
Put the 90 at "scrollxleft + 90" to a little lower number, to put the "scroll-wall" exactly on the left screen, so the player doesn't see it.
edit: reuploaded the .cap, fixed a small bug^^ You can also check the option "set invisible on start" of the wall. And the condition "player is walking" can be deleted. Dunno why I added it.