Hi Marvin, you need to make sure you have the latest LIVE version of the firmware, which i think is 4.3.2 or something.
You then have to follow the video above and log into your profile to purchase the stems upgrade from the website. You then need to make sure your device is logged into your profile and the stems features should appear.
You will then need to analyse all the music you want to use Stems with in the desktop app, and export that music to a drive that you plug into the Numark, being mindful that stems drastically increases the amount of space used on the drive, and also being mindful not to try and analyse too many files at once as it can lock your computer up.