Firstly you have to have the deluxe version so you can install the SDK. (see below)
To get the object placement tool to work you must follow this...
Taken from the Abacus Website...
"To make scenery with Scenery Shortcut, you'll have to install the SDK (Software Development Kit) that is part of Flight Simulator X Deluxe Edition. If you haven't done so already, insert DVD into your CD drive and run setup.exe from the \SDK folder.
After you install the SDK you'll have to edit a file to activate the Mission Builder's Object Placement Tool. The name of the file is dll.xml and you'll find it in this folder:
C:\Documents and Settings\username\Application Data\Microsoft\FSX\
Use wordpad or notepad to edit the file. Find the group of statements that are identified with
Object Placement Tool. If not already changed, you'll want to change the
True so that it reads
False.
Below are the group of statements which activate (or deactivate) the Object Placement Tool when FSX is started.
Object Placement Tool
False
False
..\Microsoft Flight Simulator X SDK\SDK\Mission Creation Kit\object_placement.dll
After you make the change to the file, save it. You're now ready to design new scenery."
The path to where the SDK is installed is crucial. I tried it and had the same problem until I corrected the path.
Should work fine if you follow the instructions to the letter.
Hope this helps