Error 1 The type or namespace name 'ConfigurationManager' does not exist

The most likely this issue is a missing a reference to the System.Configuration.dll. I had the issue about this when i was working on my Data Access Layer in my project. Eventhough i have declared the "using System.Configuration" namespace. it still didnt work. So i decided to do an Add reference to the System.Configuration.dll at my Class Library Data Access Layer project, then it worked. Hope this helps