Hi,
First thing, you mention UDK and I want to make mention that the AnswerHub is specifically setup for issues regarding Unreal Engine 4.
Regarding FBX 2013 - UE4 uses FBX 2014 now.
Something to check on your model is to make sure that all your Normals are facing the correct way for your mesh. If you have any flipped faces this will cause the mesh to not be visible since the back face is being culled out.
If you’re still having issues make sure to post a screen shot.
Thank you!
Tim