In the My Media view, did you have your iMovie library name selected in the sidebar, as shown in the screen shot that I previously posted? When you click on the iMovie library name then the photos and videos that are actually in the system will be displayed in the media pane. From there they can be sorted by View/Sort Clips By.
However, you cannot sort from the Photos library sidebar item because the photos displayed when you click on Photos are not actually in the iMovie system. The photos that are displayed in iMovie are contained in the Photos app library and iMovie just refers to them. They haven't been imported yet, so Sort By won't work on them and will be grayed out. When you drag one of the photos into an iMovie timeline, then it it will enter into the iMovie system (the iMovie library).
If View/Sort Clips By is still grayed out, then you might try redownloading iMovie from your account purchases in the app store. First drag your present iMovie app (the app, not the library) from the Applications folder into the trash, but do not empty the trash. Then, if you cannot successfully get a download from the account purchases window in the app store, you can drag your iMovie app out of the trash and replace it back in the Applications folder.
-- Rich