You are falling because there is no mesh beneath you.
If you hit the "F" key to turn on Fly mode (a small plane icon should appear in the top right of your screen), you can then fly around and change height and fly thru things. Use this mode to navigate your way to a part of the model where you are above a slab or mesh surface. Hit "F" again to turn off.
When you are not in Fly mode (no plane icon) you are constrained to a certain height above whatever you are positioned over, and you will not be able to go thru walls or other obstructions. Use this mode to move around the building or site, being careful not to fall off the edge of the earth.
If you hit the ESC key a menu will pop up where you can find other key shortcuts and settings.
Hope this makes sense.