Sorry about that. It's common in examples on the Internet and in books to include only the code of central interest so things are sometimes dropped and you need to download the example or do some research to fill in the gaps.
In this case, there should be a using statement at the top of the file to include the System.Globalization namespace. So add this to the top of the file:
using System.Globalization;
Then I think you'll be okay.