With these models you will need to attach your monitor or television to the bracket whilst it is mounted on the wall or desk, and then adjust the spring tension. The gas spring is made to hold a large amount of weight so needs to be anchored before adjusting.
You should find that with the weight of the monitor/TV, turning the allen key toward the minus symbol (clockwise) will reduce tension, which will eventually release the spring and the monitor/TV will slowly drop to desired height. Then simply turn the allen key back the other way (anti-clockwise) to set in place.