I've figured it out! I shouldn't have created a new config file. There is a default app.config file in the project. Now everything is fine.
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
</configSections>
<connectionStrings>
<add name="Trackboard.Properties.Settings.TrackboardConnectionString"
connectionString="Data Source=(LocalDB)\v11.0;AttachDbFilename=|DataDirectory|\DATABASE\Trackboard.mdf;Integrated Security=True"
providerName="System.Data.SqlClient" />
</connectionStrings>
</configuration>
private static string ConnStr = ConfigurationManager.ConnectionStrings["Trackboard.Properties.Settings.TrackboardConnectionString"].ConnectionString;