To mount a custom ISO, go to Services --> My Services and select your service to which you want to mount your ISO. Then, scroll down, and click the pencil icon or "change" next to the "ISO" text. Here, you can pick "Custom ISO" which will take you to a page where you can make the request. Be sure to fill out the "rDNS Domain / ISO URL" field. Only enter a URL that is direct, meaning a link to the ISO. Please also make sure it is under 2GB.