Open GoZync.fp7 from your host or FileMaker Server and click the button to log in as an administrator. (The admin password was on that layout, though you've hopefully changed it and/or removed it from the layout.)
Once logged in you'll see a button in the green bar toward the bottom half of the screen that reads "View & Enter Additional Licenses". Click that and you'll see a screen where you can manage your licenses.
Each additional license you enter counts toward your total number of licensed devices, so if you only have two licensed (as shown below)...
...and need a total of 5, you can purchase 3 more by clicking "Purchase Licenses". Once you've purchased your licenses you'll be emailed a license key within a few minutes.
When you've received your license key, click the green plus button beside "Add new licenses" and fill in the fields provided. You'll need the license key, register-to name, and order number provided in the email you received to confirm your order.
That's it. You've added a few new devices to your license. You can come back any time and add new licenses.