UPDATE: See below, there's an easier way to do this. Plus DirectAdmin's way doesn't seem to be working through the 'Custom HTTPD Configurations' way.... So, just use the way listed below. It's much easier.
*** OLD WAY / start ***
I work with Drupal's Domain Access module a lot. However, if you're using DirectAdmin as your hosting control panel, then making sub-domains will not work with the default setup. DirectAdmin puts the sub-domain into a separate directory, and in order for Domain Access to work with sub-domains on DirectAdmin, this needs to be set as the same directory/path as the top domain.
A simple work around is to login to DirectAdmin as administrator, and go into "Custom HTTPD Configurations".
Here you will paste the following code: