There is a setting in SellUp Global Settings to where it will display. We have set the best setting as default for most stores but you can change the location setting to be above or below the add to cart box.
You can manually position SellUp on the cart page by inserting the following script into your Themes Product.liquid or the relevant Snippet which displays product sections:
For merchants on Paid Plans, we can carry this change out for you. Simply send us a message along with where you wish the upsell to be located.