I don't think there is a 1-click option from query builder to Create MV, but you can always CTRL+C to copy the SQL from query builder to clipboard, then open the Create MV window, then CTRL+V to paste the SQL from clipboard into the Create MV dialog.
if you are creating a lot of MVs you could add a button on the toolbar to launch the Create MV window like this:
If you feel like there is too much stuff on the toolbar, see this post on simplifying Toad's GUI.