From the left menu select Settings/Licences and then click the Register button. Here you can add any licences you have purchased or trial licences for evaulation.
The Licence screen lets you view, add or remove a licence. DBmarlin comes with 1 free forever licence which is shown on this screen. Any additional licences will be listed along with their properties such as their expiry date, number of database targets included and the hardware MAC address of the DBmarlin server which the licence is tied to. Licences close to expiring will show in amber and any which have expired will show in red. Licenses are required for database instance monitoring but not for host monitoring.