i rented a movie in itunes how do i get back to it to watch it

You have 30 days to start the movie after it is rented, the moment you start watching the movie you then have 24 hours to finish it. So for example if you start a movie at 7:30pm on Monday night you have until 7:29:59pm on Tuesday night to finish it.

http://support.apple.com/kb/HT1657

Once that 24 hour window closes you'd need to re-rent (and pay) to finish the movie.

Yes but do you have to click the rent movie tab again to keep on watching it?

If it's been longer than 24 hours since you first started watching it, yes you will need to rent the movie again to watch the movie.

no it hasn't been longer than 24 hours. I'm using my macbook pro