How do you use the COnfigurationManager class to read and write to the settings file?
check the following
http://msdn.microsoft.com/en-us/library/system.configuration.configurationmanager.aspx
http://www.codeproject.com/KB/cs/SystemConfiguration.aspx
One thing that might catch you out is that to access the ConfigurationManager class you have to manually add a reference to System.Configuration. The System.Configuration namespace is split across multiple assemblies - you only get part of it by default.
Space Cracker posted some good links, but there is also a way to write strongly typed configuration sections, which I blogged about a few years ago - Configuring a .NET 2.0 Application Using the ConfigurationSection class. I've literally never seen anyone else use strongly typed ConfigurationSections, but once you get used to them they're quite useful.
I could try and give you the breakdown here but really the following links, all written by Jon Rista, are the best resources I have found regarding the System.Configuration namespace.
Unravelling the Mysteries of .NET 2.0 Configuration