The Agent key code is the same as your RE/MAX Mainstreet Username. Your Mainstreet Username is the portion of your remax.net email address that appears before the @ symbol. (i.e. firstname.lastname@example.org johndoe is the Username.)
You can also view your Agent key code by logging into your Mainstreet account, selecting the 'Marketing' drop down box, and then choosing 'Agent Branding for Mobile'.