If you try to add a google calendar and see an error message like this before you even get a chance to login:
Then it's likely that a popup blocker, or other content blocking mechanism has prevented Marvin from connecting to your google account.
If you're using Firefox, the built-in tracker blocker may be causing this issue:
First click the button at the top-left of your screen next to the URL input, then click "Turn off Blocking for This Site" in the popup.