While I am no expert I think that mesh can be a tricky thing. The problem is you have to define the area that the mesh will go in get it wrong and you can get the effects that you see here. I would suspect that it is not placed correctly within the original scenery and the problem is not something that can be fixed without editing the mesh itself. Now mind you I have nothing against good mesh it can make a difference but as the name applies it is supposed to mesh or blend in with the existing scenery and all it takes is to get one thing wrong and it can appear to float or even drift. So the problem more than likely has nothing to do with how you install it but it is with the mesh file itself and how it is described. Sometimes you will see what is called an exclude file as part of the mesh which removes a described area to allow the mesh to fit into it. But this is not always needed.
my advice is to find a different mesh as trying to correct this can be a hair pulling incident. Myself I do not use a lot of mesh just for that reason. Mesh is a tricky subject and has to be defined just right or problems will occur.
Sometimes the order that you display the scenery can cause the same effect so you might want to put the mesh file below the original scenery and not the other way around.
I am sure you have noticed that FSX has a scenery cfg file which describes the order that the scenery is displayed and has a priority. One of the nasty thing that FSX does is any new scenery is always put at the top of the list with a priority of 1. Usually this does not make a difference but in some cases in can so try putting the mesh at the very bottom of the list and see what happens. Granted it might not do anything but it is worth a shot.
