My suggestion is to go through the Multiclip tutorial provided with Studio or FCP 5. It has both media and instruction for learning Multiclip editing.
A Multiclip is a psudo clip of the real clips in the browser. The Multiclip doesn't really exist except as a concept shown in the timeline.
If each clip can play in the Viewer then I doubt your media is corrupted.
The "Operation Not Alowed" is like saying " There is something Wrong with Your System - Have A Nice Day!"