What I do is to make a folder in the addon scenery folder called MESH than within that folder I make another folder with the name of the mesh { ex If I was putting mesh for atlanta I would use atlanta for the folders name by doing this I wont forget what each mesh is for}. Then inside the atlanta folder I make a folder called Scenery and put the bgls there.
  Once you do this start FS go to the scenery icon and add the scenery to FS . Once you add the scenery you will need to restart FS in order for it to take effect when FS starts you should get a message about adding new scenery If you do not then recheck your scenery files to see if it is there. 8)

It would depenend on the mesh. You might want to check your settings they may be to low to see the mesh.
You might also have to change the layer it is in as well.
Any scenery you add will always become layer 1 by default if there is default scenary for this area the mesh should be set at a level below the default level.
I usually put any mesh scenery in the scenery.cfg file at the very end of the file just add 1 to the last area and to the last level of the last line. By doing this you make sure that the mesh is loaded after everything else.
If you are not sure what I mean drop me a line and I will explain. 8)