What kind of materials are you loading to the mesh?
In any case, for hard to reach mesh you can open material as following:
1. For iMtl you open it at the top of Materials List section
2. For .sbsar substance, open it from Substance section in Materials:
You may also select the Material with Pick Material tool (Shortcut is key B)
And then replicate (if nessesary) currently selected material to other meshes with Paint Bucket icon
Note: If you have a mesh with inverted normals, Paint Bucket and Pick Material Tool would not select/apply the material (Example - a back face of the plane)