You can put your widget on almost any page in your shop. You just have to find some HTML code and paste our code inside.
The following Step by Step Guide was created using an generic Wordpress site as an example (While you maybe using a different builder the same principles apply):
Login to your Wordpress admin site and click on pages (ususally located at yousite.com/wp-admin)
2. Go to the page you want to install your widget on. Click on text view (could be called html/code view)
3. In another browser tab/window navigate to paywhirl.com and login to your dashboard.
4. Create a New Widget and select your plans and upsells (if you want upsells)
5. Save Widget and Click Edit Widget
6. Now you will see the code you need to copy into you site.
7. Go Back to your WordPress admin tool and paste the code inline with where you want it to show up on your site.
8. Save this page by Clicking the Update button on the right. Now when you View your Page your widget will be embeded!
Check out some the support docs for ways to customize your widget!
NOTE: You might need to create a new page on your website so you can get to an editor.