You must press the "Redeem" button in your account under Products>Your registered hardware device. This will add Ableton Live Lite to your account if your hardware qualifies.
You will find Ableton Live Lite in your my.presonus.com account under Products>Add-ons if you qualify for the software according to the hardware that you have registered. Simply copy the serial number and click the "Download Installer" button to be taken to Ableton's web site where you can register the code.
Ableton support page: https://www.ableton.com/en/contact-us/