Configuring preferences and options in Outlook.com
I was having the same problem as everyone that couldn't find a solution. I am using gmail and google calendar with NEW Outlook on my Windows machine. It would not let me add an ICS file in any way. It would not let me select a calendar when doing all of the provided solutions. I finally went to my native google calendar at calendar.google.com and was able to add there. It's still not a great experience in native Gcal. You have to go to your calendar, click the settings gear in the upper right corner, go to import/export, and then select the ICS file from your computer. This will add it to google calendar and then because Outlook is pulling the data from your google account it will show up in the outlook calendar.
When reviewing online it seems new outlook doesn't play nice with ICS files. The problem appears to not be the drop down menu to select a calendar but the fact it doesn't actually populate the ICS file when it opens the import dialog UX. I doesn't think you have even selected a file to import yet so it doesn't give you the option to select a calendar to add it to. This makes you feel like the drop down menu is broken, when in fact, it is pending a file in order to give you a selection.
So, the only work around I can find is to open native Google Calendar in a web browser and import it there.
I hope they fix this in the future.