If you have not yet created a Certificate Signing Request (CSR) and ordered your certificate, see
SSL Certificates CSR Creation :: WebSite Pro.
How to install your SSL Digital Certificate to a Website Pro 3.x Server
Download your Root (TrustedRoot.crt), Intermediate (DigiCertCA.crt), and Primary Certificate (your_domain_name.crt) files from your DigiCert Web-PKI Customer Account to the directory where you will keep your certificate and key files.
Open the Website Server Properties.
Go to the Key Ring tab.
Click the Add Trusted Root button. Choose to install your Root Certificate File (TrustedRoot.crt)
Click the Add Key Pair & Certificate button. Here you will install the Intermediate/CA certificate (DigiCertCA.crt) and the Primary Server Certificate (your_domain_name.crt)