public void Zip(string path, string outputPath)
    {
        using (ZipFile zip = new ZipFile())
        {
            zip.AddDirectory(path);
            zip.Password = "password";
            zip.Save(outputPath);
        }
    }

using (ZipFile zip = new ZipFile())
{
    zip.Password = "password";
    zip.AddDirectory(path);
    zip.Save(outputPath);
}

using (ZipFile zip = new ZipFile())
{
    //this won't be password protected
    zip.AddDirectory(unprotectedPath);
    zip.Password = "password";
    //...but this will be password protected
    zip.AddDirectory(path);
    zip.Save(outputPath);
}