Don't kneelYour bottom should still carry the bulk of your weight, and the knee pad is there to support your shins. Begin by sitting, bottom on your seat first, and then moving into the kneeling position afterward. After your shins are in place on the kneepads, the bulk of your weight should be on your bottom. Keep in mind Peter Opsvik's concept of variation and movement: it's essential to vary your seating position frequently during the day.
Allow a week or two to get used to it
Your body will need time to adapt to its new open posture seating position as your muscles and bones have gotten used to a particular seating position. Therefore, it may feel awkward sitting in this new position if you have been sitting mainly on traditional chairs before. It may be a good idea to try some regular exercises to rebuild your strength.