Login to your ThriveCart account and select "Get URL" on your product
Copy your ThriveCart URL, e.g. sslcheckout.thrivecart.com
Login to your ConvertBar account and create a new site using the ThriveCart URL above which you have just copied. See this tutorial on how to add a site to you account.
In the Bar Settings under "Pages to display this bar on" add your ThriveCart page name.
The page name can be found under "Get URL" on the product in your ThriveCart account.
Copy your ConvertBar embed code. See this tutorial on how to locate your embed code.
Login to your ThriveCart account and on your chosen product go to Edit > Customers > Tracking and then paste your ConvertBar embed code into the middle box, then click Save & Get URL.
Your ConvertBar will now display on your ThriveCart checkout page.