Sorry, Visual Studio's designer doesn't work with MGUI and I'm not sure if it'd even be possible to get it working since would involve an entire MonoGame process inside it. The Visual Studio XAML editor is still useful for various intellisense like autocomplete or syntax highlighting.

If you're working on a simple UI, then in my opinion the best approach is to use the MGXAMLDesigner control. If you run the Samples project, the Compendium window has a 'XAML Designer' window under the Dialogs section. Open that window at runtime, and browse for a file with the 'From File' tab. I usually edit my XAML in Visual Studio's XAML Editor (to utilize the autocomplete/syntax highlighting etc), then click the Refresh button on the MGXAMLDesigner control to see how it looks. The MGXAMLDesigner doesn't support DataContexts yet but would probably be a simple feature to add, or you could just hardcode a DataContext in MGUI.Core.UI.MGXAMLDesigner.RefreshParsedContent() while working on a dialog.

Of course, even with the above approach you'll still only be able to see how the result looks. There's no properties window or anything that lets you manually place controls with the mouse cursor etc. I probably could add some kind of properties window to the MGXAMLDesigner control though.