I work in the Fitness Industry and we threw all ours out, as Ab Cradles have been found to put strain on the Cervical Spine, as most people use them incorrectly.
Most people who use these things do not engage their abs to raise the shoulders, they just pull the neck forwards, by pushing against the frame. The one you show in the photo, is quite primitive, compared to the newer versions and doesn't have the elbow rests.
The correct way to use it, if you must is as follows:
Lie on back, head on the headrest, knees bent, feet flat on the floor.
Place the palm of one hand onto the back of the other and place them against the inside edge of the cradle, at the highest point.
Your hands are only there for stability, you should not be pushing against it.
Ensure you keep your chin off your neck, keep looking up at the ceiling. Pull your belly button in towards your spine, but keep breathing, this engages your Transversus Abdomini.
Now raise your shoulders, crunching the stomach muscles, hold the up position for about a second and return to starting point. Take a breath before you start and breathe out on the way up, then in as you return to start. Do 2-3 sets of 15 to 20 repetitions.
You can also work your obliques and lower abdominals by either dropping the knees to one side or for lower abs, raise the legs in the air.
I hope that made sense.