Nobody ever replied to this? I'm a Blender noob, but I'll try anyways, so, feel free to correct me on how wrong I am.
AFAIK, if you've got more than one material, it means you've got multiple objects, which isn't allowed. Combine your meshes into a single object, by selected them all and then Ctrl+J to join the meshes.
You'll probably have to edit your mesh a bit, making new faces to actually connect any separate pieces.
Also, you'll have to convert any quads to triangles (3 vertex faces). In 'Edit mode', select all and press Ctrl+T.
See my article '
Blender: Weapon Models for Trem' for a rough overview of the rest of the process.