Refunds are done through your payment gateway.  You will need to contact the Payment Gateway used on your Registration form to have the refund processed.

To see the gateway used on your Registration:
  1. Navigate to onMessage
  2. Select Approval > Event Registration
  3. Click Edit to the right of the Registration form
  4. Under General Info, scroll to the bottom under Payment Options
  5. The Account To Use drop down will show the description of the Payment Gateway
  6. Navigate to onMessage > Settings
  7. Select Event Registration
  8. Click the Payment Processing tab
  9. This will show you the Payment Gateway tied to the description in the Registration
  10. This is the gateway you will want to contact to process a refund