How to add html code to a product:

1) Under "Products" Menu on left side of builder

2) Select "Products" 

3) Double click on the product you want to add html code to

4) Scroll down to "Description"

5) Click on the "Source" button, this will show you the description as it is coded

6) Add your html code within description box

7) Click on the "Source button again, this will show you the description as it is seen on the site

Make sure to publish after you have completed your changes.