Adding a new entry in the config in code

[venerable bede]

I have the following code which takes in a string representing an element in my settings/config file. It then returns the value of that element.

public string GetAppConfigValue(string SettingName)
{
            string sContents;
            ConfigurationSectionGroup grpApp = config.GetSectionGroup("applicationSettings");
            ClientSettingsSection secPropSet = (ClientSettingsSection)grpApp.Sections[sSettingsSection];
            secPropSet.SectionInformation.ForceSave = true;
            SettingElement elm = secPropSet.Settings.Get(SettingName);
            if (elm == null)
            {
                sContents = SettingName + " is missing!";
            }
            else
            {
                sContents = elm.Value.ValueXml.InnerText.Trim();
            }
            return sContents;
}

This all works fine but what I need to do is create this setting with a string "NOT AVAILABLE"if it is not there.

I think its something along the lines off :

SettingElement newElem = new SettingElement(SettingName, SettingsSerializeAs.String);
secPropSet.Settings.Add(newElem);
//How do i add "NOT AVAILABLE" to its value

The above code adds this to the config "<setting name="NEW_ATTRIBUTE" serializeAs="String" />" but it has no value and no closing tag " </setting>"
Man am I ever confused :-(