I think I am having the same issue and I didn't get a nibble on my posting about it. If we are on F3Dv5, we are severely restricted on what radio buttons we can use - most are disabled. I would have to use the airport reference point altitude, but ADE_GP seems to want the "local elevation" option only. I can't even see mine, but I think the OP and I are experiencing the same thing and it is related to altitude. If I create a scenery object library and place one of those objects (which is from a MDL file), then I can see it just fine. However, everything I can read here and in the ADE_GP manual gives me the impression that the options we are given for P3Dv5 take the approach of compiling our GPs as models and then placing them into the GP BGL. I have an airport background, which should be serving as a flatten and ADE is even creating an airport elevation adjustment stub file and placing that within the P3Dv5 installation's scenery files. However, I just can't see the GP at all whereas many, including jyarddog, report that if they use the top-down view and slew up, they can see it.
In all cases, this feels like an elevation problem and I find that almost ALL P3Dv5 airports, when they are flattened, are on plateaus even with a default install. Since the documentation says "will create at local elevation," I unfortunately don't think this is a reference to the ARP, which I wish would be used, but the actual local terrain elevation. I am doing all of this with a stock install (no scenery addons except for the one I am testing). If more of the options in the ADE_GP compile dialog were available to us, it would help. Even if I do an FS8 style GP, I can't see it. My next step would be to try to create a poly in blender, with a simple square with a color on it, and compile it directly to see if this improves.
Sorry if this seems like a post hi-jack, but I believe I am talking about the same thing that jyarddog is with the exception that I can't make it work great anywhere.