There are many ways to add DotNetZip file in your project. I have mention only two ways below:
Way 1:
1. In Visual Studio go to- Tools->NuGet Package Manager -> Package Manager Console
2. Select your project from Default Project drop-down in Package Manager Console.
3. Type Install-Package DotNetZip and press enter.
Automatically the package will be included in your project.
Note: Sometime it does not work as expected.
Way 2:
1. In Solution Explorer go to your Project, then References folder.
2. Right click on References folder -> Manage NuGet Packages.
3. On the pop up window select Online from left menu.
4. Type DotNetZip on Seach text box.
5. From loaded list click Install button.
Manage NugGet Package PopUp window