Try going to manage attachments, uploading it that way...
IIRC, there's a button to push in there to actually attach it to your post. I wonder if somehow you're uploading it, but it's not attaching it? I don't have any videos on my computer to try, but I remember playing with it when they originally implemented it.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry