How do you watch Netflix on your computer?
If you're having technical problems with the Instant Watching feature, you need to contact Netflix Customer Support. They will be able to provide much better help than you will find here on WikiAnswers.
If you're just asking how it works, because you don't have Netflix yet, then: it's pretty straightforward. If a movie is available for Instant Watching, you click "Play Now" and, unless there are some technical problems, the movie will start.