How do you grant access to the Form action found on the Master/Detail section?
There is none. Access is only granted to the Admin group. The Form action is only visible to members in the Admin group. The Form link is a legacy feature where you could add Form Reports in the Form Builder, specifically for Administrators to be able to report on objects in the system. In the Application Building user guide, we should point out in the Form Builder, under the Form Reports, that those Form Reports will be visible to Admin users via the Form link in portal sections.