To add the TrustedSite floating trustmark to your OpenCart 2 or 3 site:
Download the TrustedSite app from OpenCart. It will save as a zip file, upload this to your store.
In the OpenCart dashboard, navigate to the extension installer (it should look like a puzzle piece) and upload the zip file.
Once done, you can click on the modify extension button to be linked to the trustedsite.com portal, where you can make an account and/or add your website if they haven't already.
You should now see the floating trustmark on your website.
If the trustmark fails to appear in 3.x, try clearing the store caches (using the blue gear icon in the top right of the main OpenCart admin page). If TrustedSite doesn't appear on the layout edit page, try going into admin settings and making sure all administrator user permissions are checked.