Honestly, this is not a MAST bug. This is a Unity question on how to import OBJ models. Just have to drag and drop them into a folder in Unity. The materials are just plain colors and are embedded within the models. It is possible to mess up the materials if you try to change them in the model itself, and need to reimport them.
Just use MAST to create the prefabs instead. If you tried and stopped the process, then delete the subfolders MAST created inside your model folder and do it again. Good luck.