Get connection string from App.config

Can't you just do the following:

var connection = 
    System.Configuration.ConfigurationManager.
    ConnectionStrings["Test"].ConnectionString;

Your assembly also needs a reference to System.Configuration.dll

Since this is very common question I have prepared some screen shots from Visual Studio to make it easy to follow in 4 simple steps.

string str = Properties.Settings.Default.myConnectionString; 

Also check that you've included the System.Configuration dll under your references. Without it, you won't have access to the ConfigurationManager class in the System.Configuration namespace.