What you need to include in the usings list is System.Security.Cryptography and System.Text and you are good to go. Making the functions static is just one of the best practices.
FYI, here is the controversy.
As you can see, both functions are quite similar except for precisely 2 lines. The Clear() function on both occasions are required because MD5CryptoServiceProvider and TripleDESCryptoServiceProvider classes are "managed wrappers around unmanaged resources". Putting the Encrypt and Decrypt functions to use in your application would look like this:
Just make sure that you use the same key in both the lines, wherever the lines may be (they may be in two different applications referencing a common dll containing these functions) and the same hashing parameter as well. Happy encryption!