Try the delete preferences procedure again. Sometimes it works the second time.
If still no luck, try a redownload of iMovie. To do that drag your iMovie app from the Applications folder to the trash, but don't empty the trash. Then sign in to the app store and open your account by clicking on your name at the bottom of the app store side bar. When your account opens you will see the purchases that you have made, including iMovie. See if you can download it from there. If for some reason the download is unavailable or unsuccessful, drag your iMovie app out of the trash and put it back into the Applications folder. The download should be successful, however. If so, see if the opacity bar appears.
Sometimes computers in regions other than UK or USA, or that are using languages other than English, can have some features unavailable. If that is your case you might try changing your computer language to English.
-- Rich