I had this once with an glow texture and than I fixed it by setting it in Blender under Influence (go to the texture attached to the material and scroll down till you see Influence) and either mark it as 'Color' for standard/diffuse textures or 'Normal' for normals and glow textures etc. Iirc that forces it to be visible in MED's import window.

If that isn't it than post a download link to your model here or post some screenshots cause this is a bit a shot in the dark.