First to add Application configuration file into your solution & add the following in between <configuration> tag
<appSettings>
<add key="Your connection name" value="Your connection string"/>
</appSettings>
For Access connection string follow the below link.
http://www.connectionstrings.com/access[
^]
Add
using System.Configuration;
namespace in to your solution.
getting connection string from App.config
string connection_string=ConfigurationSettings.AppSettings["your connection name"]