If you want to always have your events synced with Marvin, set up calendar sync instead.
The import feature is just a one time import.
Importing existing events
Go to this website and sign in with your google account.
Select your calendar you want to import events from at the top and set the date format to this:
Don't check any other boxes.
Download the CSV file and upload it to Marvin in account -> import.
Importing multiple calendars
You can select each calendar separately to import them into different subcategories in Marvin.
To do this change the name of the "Events" folder that gets created after each upload to the name of your calendar (e.g. Personal, Work etc).
In the end you can create a new main category called "Events" and drag all the other categories in there to create subcategories for each calendar.