I missed that you were using VS2012 (even though you clearly listed it). At any rate, you are in the correct area to add the reference to the assembly.

Not sure what msdn article you are referring to, but perhaps the namespace it lists is wrong?

The namespace and assembly you need is decided based on whether your app is a winforms app, wpf or Silverlight app.

Search bing or google for "RichTextBox winforms msdn", "RichTextBox wpf msdn", or "RichTextBox Silverlight msdn".

In the search results, look for the "RichTextBox Class" entry (usually gets returned at the top).

The msdn entry will specify the namespace and assembly name.

Keep in mind that you need to match these entries with the type of app you have (winforms, wpf, etc.).