It usually means that Visual Studio can't find the referenced DLL. Click on Microsoft.NodeXL.Core.dll in Solution Explorer, look at the Path property in the Properties window, and check whether the DLL exists in the folder indicated by the Path property.
If that doesn't solve the problem, do the following experiment:
1. Start a new WPF project from scratch.
2. Insert this line into the Window1 constructor: Microsoft.NodeXL.Core.IGraph oGraph = new Microsoft.NodeXL.Core.Graph();
3. Add a reference to Microsoft.NodeXL.Core.dll.
I just tried this and it compiled successfully.