If you're working on a 3D modeling or design project, you may need to import MTL (Material Template Library) or OBJ (Wavefront OBJ) files into Rhino for further editing and manipulation. Here's a step-by-step guide on how to do it:

1. Open Rhino: Launch Rhino 3D on your computer to begin the import process.

2. Navigate to File: Click on the 'File' menu at the top of the screen to access the import options.

3. Select Import: From the 'File' menu, choose the 'Import' option to open a dialog box for selecting the file you want to import.

4. Choose File Type: In the dialog box, select the file type as MTL or OBJ, depending on the file you want to import.

5. Locate File: Browse through your computer files to find the MTL or OBJ file you want to import into Rhino.

6. Click Open: Once you've selected the file, click 'Open' to begin the import process.

7. Adjust Options: Depending on the file type and your specific project needs, you may have to adjust import options such as scale, units, and coordinate system.

8. Confirm Import: After adjusting the options, confirm the import to bring the MTL or OBJ file into your Rhino workspace.

9. Edit and Manipulate: With the file imported, you can now edit, manipulate, and further refine the 3D model using Rhino's powerful tools and features.

10. Save Work: Once you're satisfied with the changes and adjustments made to the imported file, remember to save your work to ensure that the changes are preserved.

By following these steps, you can successfully import MTL and OBJ files into Rhino for your 3D modeling and design projects. Whether you're working on architecture, product design, or any other 3D project, the ability to import external files is a valuable skill to have in your toolkit.