Will ExcBuilder work to exclude trees in FSX? I'm trying to create the BGL exclude file in FS9 and then use it in FSX. Trees are not being excluded however in FSX.

You need to use the Shp2Vec tool to create exclusions for Autogen objects - trees - in FSX.From the BglComp SDK documentation:Note that if excludeAllObjects is set to true all scenery objects will be excluded, but not any objects (notably trees) created and placed by Autogen. To exclude autogen refer to the Terrain and Scenery documentation (the section on the Shp2Vec tool). Emphasis added.