first add new item by right click on solution explorer then select Application Configuration File
and type this code
<configuration>
<configsections>
</configsections>
<connectionstrings>
<add name="AccountingWindowApplication.Properties.Settings.WinPhoneAppConnectionString">
connectionString="Data Source=COMPAQ-PC\SQLEXPRESS;Initial Catalog=WinPhoneApp;Integrated Security=True"
providerName="System.Data.SqlClient" />
</add></connectionstrings>
</configuration>