I've never tried to upload anything to youtube, but I don't think there should be any plugins required. You're not trying to view the video, just send them a file. Does this happen every time, or intermittently?
Youtube finally worked in Safari! Turned out what I needed to do was drag the clip I wanted to use out of my main imovie file and upload that only. I am very new at this and was trying to upload the entire imovie file.