Open the folder with the DER certificate you have just downloaded and then double-click it:
The certificate details will appear. Click "Install certificate":
The certificate import wizard will be launched. Follow the wizard's on-screen instructions to import the certificate:
Select a storage for the certificate and click "Browse":
Select "Trusted root certification centers" and click OK:
Click "Finish":
When the security warning appears, click "Yes":
The installation is complete.