I usually post to YouTube, then use the Share link. Click the little film strip showing in the reply box (next to insert link/insert photo at the top of the text box), then add the link from YouTube and it should post.
If you haven't used YouTube before, I don't think it will actually allow you to post a really large video for your first one. You might have to compress it some. What is the file size (hover over the video in your My Pictures file or wherever it is, and the file size should pop up).