You have to follow what's in these videos that Jason made. They're tailored for SD 7.5 but you can appply them to 7.1 and 7.2 as well.
https://www-secure.symantec.com/connect/videos/servicedesk-customization-add-custom-data-overview
https://www-secure.symantec.com/connect/videos/servicedesk-customization-add-custom-data-method-1-custom-data-type-part-1
https://www-secure.symantec.com/connect/videos/servicedesk-customization-add-custom-data-method-1-custom-data-type-part-2
https://www-secure.symantec.com/connect/videos/servicedesk-customization-process-type-actions