When you receive your certificates from Positive SSL, there will be your site certificate (named yourdomain.crt) plus 2 others (UTN-USERFirst-Hardware , AddTrustUTNServerCA and PositiveSSLCA); these 3 must be installed as a certificate chain.
Sign onto the web server and select the server to manage.
Select the Security tab and then Install Certificate.
Open the UTN-USERFirst-Hardware.crt in a text editor.
Select Trusted Certificate Authority, enter the password and copy the text from the UTN-USERFirst-Hardware.crt to the Message Text box (including the BEGIN and END lines), then click OK.
Accept the certificate.
NOTE: Do not shut down or restart the server until all steps have been completed.
Repeat the above steps using the text first from the AddTrustUTNServerCA.crt and then the PositiveSSLCA.crt and select "Server Certificate Chain" option as opposed to Trusted Certificate Authority.
For the site certificate again, repeat the steps from * above, but this time choosing This Server instead of Server Certificate Chain. At this stage all the certificates are installed and SSL now needs to be activated.
Select the Preferences tab and then Encryption On/Off.
Set encryption to 'On' and Port to 443, click OK, then Save and Apply.
Now shutdown and restart the server.