Once logged into the Control Panel in the web app, click My Devices on the left sidebar. This will take you to a list of all your account’s devices. 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 an additional 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.