First Steps > View Localhost
Your default host for MAMP PRO is “localhost”. The files for localhost are initially located in the “/Applications/MAMP/htdocs” folder. Press 
located to the right of the “Name” text box to open up the localhost in a web browser.
Note: Although the default localhost document root is located in "/Applications/MAMP/htdocs", it is best to keep it's document root, and the document root of additional hosts out of the "/Applications/MAMP" folder. A better directory structure for your host document root folders would be "~/sites/localhost", "~/sites/site1", "~/sites/site2", etc ... .