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1 & 3 have the same answer, so I'll just make that 1.
1) Pressing "F" enters floor mode, which you can use to paint the floor of the current room segment you have selected, or right click to delete the floor. (You would make 2 rooms on top of eachother and delete the floor on the second floor.)
2) X10 is only for Windows Vista, but it does have some major benifits. Ex. Water, DarkAI(Smarter AI and allies), Bloom, and other graphical enhancements. The Free Version of FPSC (FPS Creator) X9 doesn't have an of those and won't let you compile your levels into a finished executable.
You pretty much have two choices if you have Vista:
1) Buy X10
2) Buy X9 and wait for the Migration
http://forum.thegamecreators.com/?m=forum_view&b=21&t=142497&p=0
If you have any other questions, feel free to ask.
Best of Luck,
Crusader2
Some people say that I'm crazy... And I agree.
USE PLY'S MOD!