Check the box " Use Clickbank "
Insert your ClickBank Nickname.
Insert the Clickbank Link Number for that specific product.

Login to ClickBank and insert the URL provided by JVManager into the same NUMBER of the "Thank You" page in your ClickBank account.

Notes:

ClickBank is used by JVManager as a payment system only. Commissions will not be tracked by Clickbank. 

If you want to use ClickBank anyway for both the payments and also for your affiliate program, you'd at least create a 0 price product in JVManager, include the instructions for the purchase inside the Product and direct your customers to that Package's URL after payment.

Why should you do that?

Because this way you can include your customers to JVManager. It's not wise to maintain 2 lists of customers.