To install the AlphaSSL Root Certificates, perform the following steps:
Download the AlphaSSL Root Certificates
Login to the Plesk Control Panel.
Select 'Domains' from the left hand menu.
Click on the domain name that the certificate is for.
Click on the 'Certificates' menu item.
Look for the box on the page labeled 'CA Certificate'. You will need to paste both the Intermediate Certificate and Trusted Root Certificate into this box.
-----BEGIN CERTIFICATE-----
GlobalSign Root CA Certificate
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
AlphaSSL intermediate Certificate
-----END CERTIFICATE-----
Click the "Send Text" button