Your license key for Pix4Dmapper can be found on the registration card inside the documents package included with the Aero-M:
3DR Product Registration Card
Visit Pix4D here to create an account and redeem your license key; then visit the download page to download Pix4Dmapper Discovery. Pix4D will automatically upgrade from the Discovery edition to the LT 3DR Edition (or Pro Edition if you selected to upgrade) when you log in to the program with your Pix4D account information.
Click here for more information about upgrading your Aero-M to Pix4Dmapper Pro Edition.