When working with 3D models, importing MTL or OBJ files into Rhino can be a useful way to incorporate existing designs into your projects. To import an MTL or OBJ file into Rhino, follow these steps:

1. Launch Rhino and open a new or existing 3D model project.

2. Go to File > Import and select the MTL or OBJ file you want to import.

3. In the Import window, you may have the option to adjust import settings such as units, texture options, and other parameters. Make any necessary adjustments and click 'OK' to import the file.

4. Once the file is imported, you may need to make further adjustments to the scale, position, or orientation of the imported object to fit into your project as desired.

5. You can also make additional modifications to the imported object using Rhino's editing tools to refine the design and integrate it seamlessly into your project.

By following these simple steps, you can easily import MTL or OBJ files into Rhino and incorporate them into your 3D modeling projects. This can save you time and effort by leveraging existing designs and assets to enhance your work.