This guide assumes that you have a local visitor installed and setup.
Open your Visitor. Under the Configuration tab, please click Setup. If your Configuration tab is collapsed, you can open it by clicking the arrows highlighted in red on this picture
Here you want to enter the tab called License
Clear the License key field and enter your license there and press save.
you will get a pop-up asking you to restart Visitor. (the yellow part of the picture is added to distort the license used in this guide)
Close down Visitor. The next time you open Visitor the license will be active.