I would suggest using
PolyWorks instead of the official MapMaker. It doesn't have as good of a control scheme, but it's much more useful. If you
must use MapMaker, at least use
MapMakerPlus.
For MapMaker/MapMakerPlus:"Hiding scenery" is really just a matter of changing the
transparency slider on the right side of the screen before you place a piece of scenery. All the way to the left is invisible, and all the way to the right makes your scenery without any transparency.
You might also find the buttons below the slider useful as they place the scenery according to their descriptions.
For PolyWorks:Find the window titled "Palette" and look for Opacity. This is the same thing as your transparency slider, but with numbers. These numbers are expressed as percentages (100% means that the scenery is 100% visible, etc.). Then place your scenery.
If your scenery has already been placed and you want to alter the opacity, just select it with the Vertex Selection Tool (Hotkey: S) and alter its values in the "Properties" window.