Modifying app.config

Hi,
I am trying to modify my app.config file during setup using Visual Studio 2010.
The setting I want to modify is:

<setting name="SageCRMSkypePlugin_CRMWebService_WebService" serializeAs="String">
<value>http://servername/crmskype/eware.dll/WebServices/SOAP</value>
</setting>

I've tried a lot of different methods and can't get it to work, see below.
I would appreciate any help you could give

Code:

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          string targetDirectory = Context.Parameters["targetdir"];

            string param1 = Context.Parameters["Param1"];

            //string param2 = Context.Parameters["Param2"];

            //string param3 = Context.Parameters["Param3"];

            //System.Diagnostics.Debugger.Break();

            string exePath = string.Format("{0}SageCRMSkypePlugin.exe", targetDirectory);

            Configuration config = ConfigurationManager.OpenExeConfiguration(exePath);

            CRMSkypePlugin.Properties.Settings.Default["SageCRMSkypePlugin_CRMWebService_WebService"] = param1;

            //CRMSkypePlugin.Properties.Settings.Default.SageCRMSkypePlugin_CRMWebService_WebService

            CRMSkypePlugin.Properties.Settings.Default.SageCRMSkypePlugin_CRMWebService_WebService = param1;
            CRMSkypePlugin.Properties.Settings.Default.Save();
            //CRMSkypePlugin.Properties.Settings.Default.PropertyValues["SageCRMSkypePlugin_CRMWebService_WebService"] = param1;


           

            //config.AppSettings.Settings["web"].Value = param1;

            //var settings = config.GetSection("CRMSkypePlugin.Properties.Settings") as AppSettingsSection;
            //settings.Settings["SageCRMSkypePlugin_CRMWebService_WebService"].Value = param1;


         

            //CRMSkypePlugin.Properties.Settings.Default.PropertyValues["SageCRMSkypePlugin_CRMWebService_WebService"] = param1;
               
           
            //["SageCRMSkypePlugin_CRMWebService_WebService"] = param1;

            System.Configuration.ConfigurationManager.AppSettings.Set("SageCRMSkypePlugin_CRMWebService_WebService",param1);



          // System.Configuration.DefaultSettingValueAttribute
           
            //["SageCRMSkypePlugin_CRMWebService_WebService"] = param1;

            //ConfigurationSettings.AppSettings["SageCRMSkypePlugin_CRMWebService_WebService"] = param1;
            //config.AppSettings.Settings["SageCRMSkypePlugin_CRMWebService_WebService"].Value = param1;

            //config.AppSettings.Settings["Param2"].Value = param2;

            //config.AppSettings.Settings["Param3"].Value = param3;

            //config.AppSettings.Settings.Remove("SageCRMSkypePlugin_CRMWebService_WebService");
            //config.AppSettings.Settings.Add("SageCRMSkypePlugin_CRMWebService_WebService", "http://davidhendrick/crmskype/eware.dll/WebServices/SOAP");


            config.Save();
            config.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection("appSettings");

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