You must have installted the Collection Snippets using Install Snippets button on Collection Add-ons tab.
Please note if you change your theme after doing the install steps, then you will have to re-do the steps for the new theme.
Once you have installed the Snippets follow the following steps to update Product template liquid file for your theme.
Step 1: Edit Code of your theme
Step 2: Find product.liquid file under Templates.
Tip: you can use the search box to search for product template file.
Step 3: Paste Upsell Add-on code:
Copy following line of code and insert it at the end of the product.liquid file (Please note your theme's product.liquid file will likely look different from the below screen shot)
{% render 'upsell-addons-app-product-page' %}
Note: This only applies to ultimate edition
Follow Step 1 from above
Step 2 Find cart.liquid file under Templates.
Step 3: Paste Upsell Add-on code:
Copy following line of code and insert it at the end of the cart.liquid file (Please note your theme's cart.liquid file will likely look different from the below screen shot)
{% render 'upsell-addons-app-cart-page' %}
