Well, given the error message, it seems to me that you have already figured out how to not add it... This is part of the Entity Framework 6.0 Beta, isn't it?
I understand that. Usually, you would add it at the top of your namespace like, Using System.Windows.Forms; or add a reference to it in the 'References' of your project. Do you know how to do either of those?
Are you using Entity Framework? If so, System.Data.Entity.ModelConfiguration is a namespace in EntityFramework.dll assembly. So, Add EntityFramework.dll reference to your project.
Using System.Windows.Forms;or add a reference to it in the 'References' of your project. Do you know how to do either of those?