I apologize for the outburst. I'm to blame here for the lack of delicacy in formulating a respond, but it seemed you haven't read the manual by neglecting to mention it.
Here the solution:
Option 1: Go to the root directory of the image folders in the side bar, right click on that folder, choose "Find duplicates recursive ..."
Option 2:
Open find duplicates window by pessing the D key or open File menu, choose "Find duplicates ...", drag a folder from the sidebar, choose "Add content ..." from the pop up menu, the window will remain empty, because there are no duplicates found, notice the status bar at the buttom, "0 matches found in ". Now drag another folder on that window, choose from the pop up menu "Add contents ...", duplicates, if present will be displayed.
Option 3:
Right click on a folder in the sidebar, choose "Find duplicates recursive ...", drag & drop another folder, choose, "Add content ..."
Option 4:
Open a window, check the "Compare two file sets" box, bottom right side, drop a folder on the right side, on the left the folder to compare.
I have discovered these just now by try and error. After I noticed the message in the status bar the next logical step was to drop another folder. The window remains empty, because there are no duplicates to show. The manual is to blame here for not to be presice.