As VENT says you can place object using Enter: The object will snap on top of any object underneath your mouse cursor. Shift + Enter will force the object down, so if you have a set of shelves press Enter and it will jump to the top shelf. Now Press Shift + Enter and it will snap down to the shelf below.
The Page Up and Page Down move the item slowly up and down, so you can place them exactly how you like. I prefer to do this while right clicked on an already placed entity.
These handy tips and more can be found in the manual which you can access by clicking
Help -> Read Manual from inside FPSC. In fact this very question is answered on page 33 of said legendary document