There is two ways to install the package, either use the backoffice of Umbraco to install/upload the package or using the recommended way which is to install the package via NuGet.
The package can be found on NuGet.
Just install the package using the Package Manager inside Visual Studio or via the console.
We host several packages on NuGet:
To the package via the Umbraco Backoffice, navigate to the "Packages"-section and search for "Newsletter Studio" or download the installation package from the package repository on our.umbraco.com.