To place objects in midair, you must understand hexadecimal.
For all objects except Player Starts, multiply the number of units above the floor you want the object by 16, and then convert it to hex. Add 7 (For the Easy/Normal/Hard flags.
Now right click on the object and go to "Edit Raw Data". Find the bitset and type in the number.
For instance, an object 256 units above the floor would have a bitset of 0x1007
Simple enough? *beaned*