Once you've logged into GiveGab Enterprise, select the campaign your form is in from the list of available campaigns in the upper left-hand column, and then head over to the Form List tab. 

Select the form that you'd like to add installment or ongoing payments to. The type of form will dictate your next move. For donation forms, select the Donation Levels tab. For registration forms, you'll select the Registration Types tab. In either case, you'll see a section called Payment Settings:

Click the Edit button on the right-hand side of the screen, and you'll be presented with the "Edit Payment Settings" dialog box. It's here that you can select the appropriate radio button to enable or disable your payment setting choices:

From there, you can further set the parameters of the available choices. 

***Be sure to read the italicized descriptions of Installments versus Ongoing Payments. They'll help you to make the correct choice for your donors.