Porting a multi-site setup to local (MAMP) server
Unless you've got MAMP pro, or have set it up as such (which you haven't mentioned) vhosts probably won't be enabled; Apache will be looking at MAMP > htdocs for the web root, not any folder underneath it. 

Try moving the Drupal web root up one folder (i.e. directly under MAMP > htdocs) and try again