The objects need to be added to the GSL (ground Scenery Layer (?))
Probably the most straight forward way to do it- requiring no additional utilities- is using Mission Builder's built in GSL Editor. Lindsay "leroy10" Watt, who ferreted out this tool (it was hidden and unacknowledged by MS) wrote a very nice tutorial on how to use MB's GSL Editor. It is available here at SOH I believe. You can also download it from my website: click on the link in my signature, click on the "DOWNLOADS" button at the top of the page and then clcik on the link to Lindsay's MB GSL Editor Tutorial.
Chris