Hello Joel Kraft,
Thank you for interest in RadMarkupEditor.
In order to use the dialog at runtime you will need to add a reference to the 'Telerik.WinControls.RadMarkupEditor' assembly. Also you should include the namespace 'Telerik.WinControls.UI'.
We will update the documentation asap to include this information.
If you have more questions, don't hesitate to contact us again.
the Telerik team