Download the 'Root Certificate' and the 'Intermediate Certificate' (choose either DER or PEM Format - it doesn't matter)
Log in as an Administrator
In Windows Explorer, browse to the class 1 Root certificate you downloaded and right-click it, selecting Install Certificate (and click Open and Next if necessary)
Verify that the radio box labeled Place all certificates in the following store is checked and that text box says Trusted Root Certification Authorities
Click Next and then Finish. You should get a message saying the import was successful.
In Windows Explorer, browse to the class 3 Intermediate certificate you downloaded and right-click it, selecting Install Certificate (and click Open and Next if necessary)
Verify that the radio box labeled Place all certificates in the following store is checked and that text box says Intermediate Certification Authorities
Click Next and then Finish. You should get a message saying the import was successful.