Once logged into the control panel http://my.Mypcbackup.com click 'MY DEVICES' on the left side bar. This will take you to a list of all your account's devices in a table. The far right column, labelled 'License' will display which devices are licensed with a tick and which are unlicensed with an 'Add' button.
Clicking add will enable you to either transfer a license from another device or purchase a new one.
License Assigning should be almost immediate, if it isn't simply click 'Settings' on your app and then click OK - this will force communication with the server and recognize the assigned license.
You only need licenses for backup. The app can be installed on any number of your computers and you can still use the sync folder and restore files without a license.
Mobile devices such as iPhones, iPads and Android Cell Phones do not require licenses in order to be backed up.