Enter from the left menu: Tools > Rankings Discovery.
You'll get to the Rankings Discovery section, PRT will display discovered ranked terms.
The data that is displayed is for all the URLs in your account, but you can also select a group or a URL at the top to display specific discoveries.
Click on terms you want to add for tracking in the left box (of each URL) and move it to the right box, than when finished click "Add" on the right bottom of the page.
For each URL will be displayed 200 terms maximum, and you can click "View all" to view more discovered terms if there are.
It redirects to a page where you can select the URL on top right, view the discovered ranked terms and their details (rank group, device, engine, last time it was updated, etc...), select the terms using the checkboxes on the left, and add from the bottom left button.
*Switch to the Single URL mode easily from the top right.
Also, when adding new URLs/Terms from the Advanced Add or Add Wizard PRT will display discovered terms it finds.