Same here. Nice new stadia controller and Chromecast ultra but no code to make use of it.
You can contact support via https://support.google.com/stadia but they will likely tell you it may take 3-5 business days and to contact them again if you still haven't received the code by Friday.
and ( drumroll ) the package arrived and it is lovely but cannot use it without the code obviously, will try it as a normal controller when i get home