I've found that the best thing to do is to make sure you have OF open in another tab and that you're logged into your account. Keep it open the entire time Coomer is importing, which will take forever. If you're doing multiple accounts we're talking about it taking overnight. If you're opening the importer log and after a minute you're not seeing something that is extremely normal in my experience. Keep the tab open and check back in half an hour, and you should see progress.
So I used the extension above and the 3rd one down on the coomer import site is this :