You need to create a class that inherits from MaskDescriptor. Also, if you want Type validation support, you'll need to implement a public static Parse method that takes a string.
The MaskedTextBox sample app in the SDK shows this. See the IPv5 and IPv5MaskDescriptor classes.
You can download the sample from:
http://msdn2.microsoft.com/en-us/library/ka7h9fze.aspx