Advanced topic: contact support if you need additional help!
While Marvin has Outlook calendar sync, it doesn't currently have an integration with Outlook mail and tasks. But it should still be possible (though somewhat difficult) to get Outlook tasks and flagged emails imported in Marvin.
Exporting directly from Outlook can be difficult and prone to failure! Instead try following this tutorial to just choose the fields you need, then select all the rows and past them into excel.
Then in Marvin, go to ☰ → Account → Import, and choose Generic CSV. You will be prompted to download Marvin's CSV template. Now all you have to do is massage the data you pasted into excel to look like the columns in the CSV template by moving around, renaming, adding, and deleting columns. One user reported having a lot of extra carriage returns in his notes, and used notepad++ regular expression replace to clean them up.