You click on them with the Left Mouse Button, and slowly move them where you want them by dragging the mouse.

If I use them, I set the first one to the highest speed and each one after that to the next highest speed until they are all where I want them.
Ususally for take off for V1, VR, V2, 210 KIAS for flaps and slats up, and 250 KIAS for max speed below 10,000ft.

For landing, I'll set them for the max speed for each Flap setting (Flaps 5, 15, 20, 25, & 30), however, the Flap Position Indicator has the max speed for each setting on it, so I rarely use them for landing.

It's best to move your mouse forward and backward, rather than any other direction, although I never found them to move too fast for me. I tried this in FSX, FSX-SE, and P3Dv3, and it's the same in all of the sims.