When creating or editing a form, you must associate the form with a program. If your program has enrollment types, you can choose to further limit the form to contacts who are enrolled in that particular type of program.
To associate your form with an enrollment type, open up the Form Builder by click on Forms in the lefthand navigation menu, selecting a form from the list, then clicking Edit.
At the top, under the section called "Current Associated Programs", you will see a list of programs that the form is attached to. Click on the "change" link in the lower right corner of that section. Now you will be able to select the enrollment types to connect the form to.