If you have activated our Upsell Popup feature via our app's Settings, and nothing is showing up when users click on "add to cart", then the problem might be one of the following:
a) your theme's cart type might be interfering with functionality. Currently, our app works best when using a cart of type "page". In most themes, you can verify/switch the cart type of your theme by going to Online Store -> Themes -> Actions -> Customize -> Theme Settings (sometimes you will not have this and just the Cart option) -> Cart
Then, change the "Cart" from whatever you have (e.g. Drawer/Slider/MiniCheckout/Popup) to type "page". Don't forget to hit "Save" on the top right!
b) you might have an ID for your "add to cart" button different from Shopify's default (usually "AddToCart-product-template"). In this case, just get in touch with us to verify and deploy a fix we have :)