using System.Configuration;
but still the System.Configuration.ConfigurationManager.ConnectionStrings is not found in your code, looks like you need to manually add a reference to the System.Configuration.dll file.
To add the System.Configuration follow the next simple steps:
- Righ click on the references folder
- Select Add a reference
- Select Assemblies >> Framework
- Search for the System.Configuration and select it
- Click the Add button