Select Slide 5 in the slide selector by clicking on it. You will notice a red and a blue marker in the workspace, the red saying Slide 5 begin and the blue saying Slide 5 end. Drag these markers to the appropriate position in the audio and the slides will get marked. Change the position of the markers as often as you want.

Note that marking the end of Slide 5 also marks beginning of Slide 6. Similarly marking the beginning of Slide 5 also marks end of Slide 4.