1. After download, extract the .zip to your preferred location. 
2. To insert a shader into your Scene, Choose "File --> Append"
3. Navigate the the extracted .Blend File
4. Navigate to the Material section of the .blend
5. Select the node labeled "metallic - Car Paint"
6. Press "Append from Library"
7. You will now find the node group under the material section of your blend.

For further questions please contact me via email at JohnsonMartin.Contact@gmail.com