To request an rDNS to be set for your service, go to Services --> My Services and click on the service for which you would like to request the rDNS. Then click on the "rDNS" button. Here, enter your domain, click "Submit" and wait for a confirmation. If your IP address supports it, you may be able to set rDNS directly within the VPS control panel. For that, follow Video Tutorial #2
Video Tutorial #1
Video Tutorial #2