when building on msfs no errror.
In this regard, I have to suggest you might be mistaken, I'll explain why. If the model is in the PackageSources folder, FSPackagetool.exe will Build the Package, that is why you have the empty box and crosshairs, the model information has been added to the scene, but it remains invisible. This means the compiler searched for the textures, or the model and found one or the other to be inadequate, but conducted the Build anyway. To be sure, you could check the upper right corner of the Console window, to make sure the Autoset check box is selected, the message group filter to the right should read "PackageBuilder," or higher and check the red x Error field and the yellow "!" warning field that they are not "deselected" and are each surrounded by the blue box.
Anyway, right or wrong, it would be expedient to upload your Package. Bottom line, if the model appears in MCX, it is almost certainly not a Blender issue, nor an exporter issue. If the XML loads into MCX with textures applied, it does not matter the format, MCX can do the conversions and even conduct the build. This means the model is indeed, "ready," or very close.
The problem, most likely, is paths, or procedures and the uploaded Package would determine that upon examination. Without, it can be a rather lengthy back and forth, "is this there, have you placed these textures here," kind of thing. My guess is a paths situation, based on the fact that Arno has written in a pretty thorough search algorithm to find those textures. However, untextured models usually show up pink, which suggests some problem with the model itself, which raises one final consideration. Are you sure you're waiting long enough? Sometimes it takes more than a moment for my models to pop in, especially in crowded scenes. You'd want to make sure by testing other objects as well, if this is a possibility.