I'd like to enable Advanced Tools in iMovie preferences.
When I open preferences, I only see:
Using iMovie 10.0.9
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I don't recall from which version, but at some point iMovie changed its interface so that all of the advanced tools were already built into the interface, so there's no need to enable it in iMovie's preferences.
To see the various tools, just click on your clip and select the Adjust option to reveal the Advanced tools.
Update: There is no "Adjust" menu in 10.1.9 - all the tools that were there are now either hidden until you need them or distributed in various other menus.
My issue was I could not find how to flip a clip in 10.1.9: the answer turned out to be rather un-intuitive. You go to the CROP menu. There, on the right hand side of the screen, you'll see a rotate left and rotate right button.
Other menus, such as the green-screen/blue-screen options, only appear when you have one clip overlaid over another.
These are very difficult issues to search for, as the menus have changed so drastically across so many versions - I'm adding this clarification to one of the top results for my particular 10.1.9 issue, even though this will probably be useless with the next release!