I'm not sure that I read your post correctly.
In re-reading your post, are you saying that you are not getting the option to reveal the list of cities and locations at all? in other words, are you not getting the locations box shown in this screen shot?

If so, can you post a screen shot to illustrate what you are seeing, or not seeing? I am using a much older version of iMovie 10 than you are, so don't know what changes may have been made. There must be some way of entering the locations onto the map.
The maps feature can be a little tricky depending on the region and language settings on your Mac. If your Language and Region preference settings are other than USA-English, you might try changing them to that and see if it makes a difference. You can always change them back.
Another thing you can try is to delete preferences to see if that cures it. Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
-- Rich