It will be toward the end when you select how to receive your refund.
Here are a couple things to check:
If you are using a TurboTax partner affiliate link to use the software, the Refund Advance will not be available.
If you select the 5 Day Early Refund Delivery, you will not see the Refund Advance option, but you can return to the refund selection page and find all the options listed.