Bitnami Multisite Stack - Port 80 Problem

I downloaded the Bitnami Multisite Stack on my Mac and I can't get it to install. I keep getting an error saying "There has been an error.
Unable to bind to the 80 port number. WordPress Multisite only supports this port for the Apache server." What do I do? (I've seen in other sites people say the problem is Skype, but I don't have Skype). Thanks in advance.