Configuring preferences and options in Outlook.com
I have also been experiencing a bug when trying to import a .ics file to the new Outlook. The window to select the calendar field is "locked," so I cannot select any calendar and therefore cannot add the .ics event file to the calendar.
SOLUTION "HACK": I found that I can add the ICS file as an attachment to either a new event or an email, then click on the .ics file attachment which opens a window to "Add to Calendar", and this adds the event to the calendar correctly! I hope this helps someone else.