To use the full version of the package you need to purchase a license on this website. When buying the license you can provide your Workspace key to associate the license with a given Workspace - you can also leave the key-field empty and connect the licenses at a later stage.
Administrators can download and install the license via the backoffice. This will work both if you provided a Workspace key during the checkout or if you want to associate the current website with a license.
If you did configure the license during the checkout process, you can download the newsletterStudio.lic file from your license-list. After this just place the file inside the /umbraco/Licenses-folder at the of your project.
/umbraco/Licenses