1) Hover on your name on the top right hand side and click Connections. Click on "Carts" tab on the top and then Click on "Add Cart", choose 1Shopping Cart and enter the required fields, enter any name you want and then enter the Merchant ID and Merchant Key you received from One Shopping cart and Save it.

2) You will see "Notification URL", copy the URL from there and paste it in the Notification URL field in 1Shopping cart website under API Settings.

3) Now, you will see this cart while creating a New list or Editing any existing list, you have to check mark the Cart (you can also enter specific 

Product IDs/Items you want integrated in this list) and then click Save.