It sounds like you are creating the platforms outside the LAYOUT, not outside the current VIEW (or as you say, window), so they will never appear. Remember that x and y refer to the location on the layout, and a -y value will be above the layout (which is useful if you have an object you want to fall into the layout.)
To make an object just above the current view, you need to take into account the current position of the view. From the manual system expressions section:
Get the current position the view is centered on.
create object solidPlatform on layer 0 at (random(scrollx-(windowwidth/2),scrollx+(windowwidth/2)), scrolly-(windowheight/2))
It will create the platform just below the top of the current view so that you can see it. To understand this, you need to know that
Scrollx-windowwidth/2 is the position of the left edge of the view. scrollx+windowwidth/2 is the position of the right edge of the view.
scrolly-windowheight/2 is the position of the top edge of the view.
scrolly+windowheight/2 is the position of the bottom edge of the view.
Knowing that, you can make adjustments to the locations by adding or subtracting margins as below:
create object solidPlatform on layer 0 at (random(scrollx-(windowwidth/2-leftMargin),scrollx+(windowwidth/2-rightMargin)), scrolly-(windowheight/2-topMargin))
where you might set variables leftMargin=20, rightMargin=20 and topMargin=-25