This one took me months to find. I could turn on the grid, but just could not find where the guides had gone. I often use them when I create greeting cards. The guides become my fold lines so I know which quadrant to put what part into. Yes, I use PowerPoint to make my greeting cards for holidays and birthdays for all my co-workers.
On the Home ribbon, Arrange group, click Grid Settings… (or right-click off the slide and choose Grids and Guides) and select the check box for Display grid on screen (ALT+F9). You can click and drag a guide to move it, or hold down the CTRL key and drag a guide to get additional guides.
View a video demonstration.