as a matter of fact, I didn't make my Factory map transparent...
But basically the only way to change the transparency of a polygon, is to change the value of the opacity (0%-100%) in between those two numbers and PRESS ENTER as soon as you have typed the new number you want.
For example you have:
Opacity: 100% - the polygon is completely visible, but you want to change it to be completely invisible.
You type:
Opacity: 0% and as soon as you type it, you AUTOMATICALLY PRESS ENTER so the changes would take effect. Otherwise the changes won't take effect.
As soon as you press enter, the selected polygons will become completely invisible to the eye.