The stem on a D4+2.0 is adjustable in several ways. The first way to tighten down the stem is to adjust the 10mm tension nut that is located on the underside of the folding mechanism:

The key to adjusting this nut is to tighten it as much as possible while still being able to move the red lever into its upright, locked position. If you feel as if you are putting too much stress on the lever, it is likely that you need to loosen this nut. It is possible to break this mechanism from tightening it too much, so it is crucial to find the sweet spot.

The second way to reduce play in your stem is to tighten down the two main stem nuts:

Before you tighten either nut, it is recommended to use some sort of protective barrier (a rag, piece of rubber, etc.) to cover the nut. It is easy to scratch and mar the surface of these nuts, and eventually the nut will be stripped. The key to tightening these nuts is to tighten the lower (larger) nut clockwise, and the upper (smaller nut) should be tightened counter clockwise. The idea here is to create tension between the headset of the scooter and the stem, so the lower nut must press down (against the headset) while the upper nut presses up (against the stem).