ExeConfigurationFileMap fileMap = new ExeConfigurationFileMap();
fileMap.ExeConfigFilename = @"my.config";

Configuration myConfig = ConfigurationManager.OpenMappedExeConfiguration(fileMap, ConfigurationUserLevel.None);
KeyValueConfigurationCollection settings = myConfig.AppSettings.Settings;

namespace Tool.Properties
{
    class settings
    {
        static System.Configuration.KeyValueConfigurationCollection mySettings;

        public static System.Configuration.KeyValueConfigurationCollection Default
        {
            get
            { return mySettings; }
            set
            { mySettings = value; }
        }

        internal static void LadeEinstellungen(string KonfigDatei)
        {
           
            System.Configuration.Configuration config;
            System.Configuration.ExeConfigurationFileMap configFile = new System.Configuration.ExeConfigurationFileMap();
            configFile.ExeConfigFilename = KonfigDatei;
            config = System.Configuration.ConfigurationManager.OpenMappedExeConfiguration(configFile, System.Configuration.ConfigurationUserLevel.None);
            config.Save(System.Configuration.ConfigurationSaveMode.Modified);
            System.Configuration.ConfigurationManager.RefreshSection("applicationSettings");

            mySettings = config.AppSettings.Settings;
            //Tool.Default = config.AppSettings  ;
        }
    }
}

private void ButtonKonfigLaden_Click(object sender, EventArgs e)
        {
            string datei = lsbProfile.SelectedItem.ToString();
            LadeEinstellungen(datei );
            string StarterWert = Tool.Properties.settings.Default. //.... hier gibts keinen Startwert
        }