Please enter the VirtualDJ KeyCode from the card that came with the DDJ-SZ:

Info To activate your full PRO license, you will need to log in and enter your keycode inside the software,
with an active Internet connection on your computer (only the first time)
Note This license cannot be sold or transfered to someone who does not own a DDJ-SZ
(it will not work if you don't have a DDJ-SZ connected sometimes)