This is how the adjustment is done:

Loosen the two (sometimes 1, but not often) bolts on the SIDE of the stem that clamp it to the fork steerer tube.
Do a 1/4 turn on the bolt that is on the TOP of the stem. That bolt is going to provide preload to the bearing.
Hold the front brake and rock the bike again. If it is still loose, repeat the above two steps.

Keep in mind the preload bolt is not to be cranked down. That bolt only provides preload. It does not tighten the stem or anything else.