In the Mamp preferences, what is the port set to? The default is port 80 for Apache and 8889 for MySQL.
if is using port 80 for Apache, then the address:
should work to access your site.
If it is using a different port, it needs to be included in the address, for example, if the port where set to 8080:
If it is using port 80, and the localhost address is not working, you may have other software installed that is using port 80 like Skype. Try changing the Apache port to 8080, if it works when changing the port, then you have a conflict trying to use port 80