That's odd. I'm running iMovie 10.1.10 (current version). The opacity slider is there as you can see from my screen shot. It doesn't show an Opacity box that can be checked.
Try unchecking the Opacity box that you see. Does the slider then appear?
Absence of an opacity slider would indicate that iMovie is not functioning correctly. Perhaps it is a corrupt preference. Try opening iMovie while holding down the Option and Command keys and selecting 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. Now see if the opacity box appears.
Failing that, try booting up in Safe Mode (see Help menu) and, after everything finishes loading, immediately shut down and reboot in normal mode. See if that cures it.
If still no luck, try redownloading iMovie. Drag your present iMovie app from the applications folder into the trash, but do not empty the trash. Then go to the app store and see if you can redownload your version of iMovie. If you can redownload successfully, then you can empty the trash. If for some reason a redownload is not available, drag your old iMovie out of the trash and replace it in the Applications folder.
Hopefully one of the above will solve the issue.
-- Rich