To enable the recurring payment for a custom form, make sure you've completed all of the following three steps:
1) Have an Authorize.Net account with CIM activated. http://www.authorize.net/solutions/merchantsolutions/merchantservices/cim/
2) Turn on the recurring payment on your site http://www.yoursitedoname.com/settings/module/recurring_payments/#id_enabled. Set Enabled to be true.
3) When creating/editing a custom form, make sure the checkbox "Is Recurring Payment" is checked. (It is located under the section Payment.)