Drupal multi-site not working
A multisite configuration is used when a web server is associated to different domain names, and you want the web site to appear different when accessed from different domains. (If you want the web site appears the same in both the domain, there isn't any special Drupal configuration to follow.)

Suppose that your web site answers to example.com, and example.org. You can have two different settings.php files:


sites/example.com/settings.php for example.com
sites/example.org/settings.php for example.org


In the case those files are not found, Drupal would use the default one (sites/default/settings.php).

The same can be done for subdomains. 

On your Mac, you should edit your hosts file (/private/etc/hosts) to add something similar to the following one:

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting. Do not change this entry.
##
127.0.0.1 localhost
127.0.0.1 first.localhost
127.0.0.1 second.localhost


In this case, the following files would be used:


sites/first.localhost/settings.php for first.localhost
sites/second.localhost/settings.php for second.localhost


If those files are not found, Drupal would look for the following ones (and other files as well):


sites/localhost/settings.php
sites/default/settings.php