As Ken has alluded to, to export to an add-on like PMDG, or LDS etc that can only reside within the FS directory tree, you need to set the location of the FS home folder on the options/Export Directories page. Then it will automatically know the location of PMDg etc.
Then go to the settings tab, and press the 'save config options button to save the settings. Then restart FSbuild for those new settings to take effect.
See the exporting Tutorial page for more details.
http://www.fsbuild.com/fsbtut/exporting_tut.htm
Regards.
Ernie.