Currently Being ModeratedNov 24, 2012 4:13 PM (in response to JM4N3365)
Usually this is caused by entering the wrong YouTube ID and password. Use the same Signing that you use for GMail.
If that does not help, do this.
In iMovie, Share/Export Movie in the sizes you want. Save to a location you can find.
Now open YouTube in your web browser. Use the upload function in YouTube to up load your movie.