When your certificate was issued by VikingCloud, you received an email with a ZIP file attached. Within the ZIP file, there is a file which ends in .cer - this is your certificate. Extract the .cer and return to the iPlanet administration window.
Choose the "This Server" option and paste in your .cer file. Copy and paste the entire certificate - including the dashed lines at the top and bottom. Click "OK".
Go to the Preferences tab and click "Encryption On/Off". Set the encryption to "On" and specify a port number. The default SSL/TLS port is 443. Click "OK" and restart your server to apply your changes.