After launching InkLevel, first thing you need to do is selecting a printer. You will see the 'Selected printer' pull down menu at the top of the main window. When you choose the 'Select printer' at the bottom of the menu, the Printer Browser window will open.
In the Printer Browser window USB or network printers which are discovered by Bonjour will be listed. Please select a printer you want to see ink levels, and click the 'Select' button to close the window. Shared printers and IPP printers are also listed in the window, but you can't select them, because those printers can't show ink levels.
When you return to the main window, you will be able to see ink levels of the printer your seected in the main window.