If you're going to allow your customers to choose from products you offer when creating their bookings, you'll need to configure the product items first.
When adding products, they must already be added in Setup, Inventory as this is where it will pull the item from. If you have not already setup your inventory items, you should go do that now. Once added, return to the OLR setup.
Your editing view:
Your customers view when making a reservation:
Once you have a list of products, you can use the up/down arrows to re-organize your list if you wish.
Note that in the image above, the text below the "Please select", is custom. To edit this text, in your OLR settings click the "Customize Pages" tab to edit that text.