There are two distinct types of serial numbers you might need:
The most common location is a barcode sticker on the bottom of your Numark controller. Look for a number starting with a (21) 1.2.3 . Numark Cue Serial Number -UPD-
If the sticker on your controller is faded or the box is lost: There are two distinct types of serial numbers
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If your Numark product came with Virtual DJ LE (the successor to Numark Cue), download the latest version from the Virtual DJ website 1.2.2.
Use search terms like “Numark Cue serial,” “activation code,” “order confirmation.”