Open Rhino: Launch Rhino and open the project file where you want to use the new hatch pattern.

Open Document Propterties: 

Open through menu: File > Properties (Windows), File > Settings (Mac)

Open using command: Type DocumentProperties into the command line and press Enter

Import the .pat File: 

In the Hatch section of the Document Properties panel, click Import

In the file browser that opens, locate your .pat file, select it, and click Open

The hatch patterns from the .pat file will now be loaded into your document

Use the New Hatch Pattern: 

Now, when you create a hatch, the imported patten will appear in the Hatch dialog

To apply it, use Hatch from Draw menu or type the Hatch command, select your geometry, and choose the new pattern from the list

ADDITIONAL TIPS

Scaling the Pattern: Hatch patterns may appear too small or large when first applied. You can adjust the Scale option in the Hatch dialog to fit the pattern appropriately to your geometry.

Pattern Orientation: Some patterns may not align as expected with certain geometries. Use the Rotation option in the Hatch dialog to change the angle of the pattern to match your design.

Saving Patterns for Future Use: If you frequently use the same patterns, consider adding the .pat files to your Rhino templates or default settings, so they load automatically in new projects.