as above
I'll assume you'll installed the SDK, and updated to SP1a.
You need to find the dll.xml file in the <Drive>:\Documents and Settings\<user name>\Application Data\Microsoft\FSX folder. If it doesn't exist, copy it from the SDK.
Open it up in an editor and change it so it has the following text:
<Launch.Addon>
<Name>Object Placement Tool </Name>
<Disabled>False</Disabled>
<ManualLoad>False</ManualLoad>
<Path>..\Microsoft Flight Simulator X SDK\SDK\Mission Creation Kit\Object_Placement.dll</Path>
</Launch.Addon>
If it has the tool entry already, just change the Disabled tags from True to False.
Now check the path, and make sure it actually points to the tool DLL. If you have installed either the sim or the SDK in a non-standard location, you may need to change the path.

