I can not save my video on the desktop when I using the trial version.
That's correct, you can only publish to YouTube, Facebook and PowerPoint with a trial account. You need a subscription to publish a video to your computer
I subscribe as advised, so that i can save the videos on my computer but it still says the same as before: online or scribe, only. I want to save it as mp4 so that i can add it to imovie ... Can you please advise me on that?