After creating the movie in Movie Maker, go to [File -> Publish movie] or [File -> Save movie file]. Choose appropriate settings as instructed by the wizard and upload the resulting file to Youtube.
Note that [File -> Save] or [File -> Save project] will create files usable only on your computer. Youtube will not be able to process a file created in this way. [ theymos's advice column | Ask theymos A Question ]
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