Click on to the left of "Record from:". You should see the panel expanded.
Click on the "From" combo box, and select "Window".
Note the label "No window selected" under the combo box.
Now, click on the window picker 
and do not release the mouse button. Keep the mouse button down, and drag the cursor over the screen. You will see different windows under the cursor highlighted with a dotted rectangle. Move the cursor to the title bar of River Past Screen Recorder, and release the mouse button.
You should see the text "River Past Screen Recorder ..." under the combo box.
Since we are going to capture this window, let's clear the "Minimize when capturing" check box under the main status window.
