When you are setting up the WPMU on single WordPress setup following things should be very initiated :

1 – Setup the Network for Multisite by updating the wp-config, simply add the bellowed code before “Happy Blogging” which you can find it in the wp-config.php

Code : define(‘MULTISITE’, true);

Note: once you do this step your Network Administrator will be active

2 Add the Plugin –  WordPress MU Domain Mapping –
You need to move sunrise.php to wp-content folder from wp-content/plugin/wpmu

3 Now add the following code before the happy Bloging to get the validate the plugin.

define( 'SUNRISE', 'on' ); [Before the Happy Blogging]

4 – Add the domain to you web hosting control panel in the Parked domain

5 – Add the same site to the Newtork Dashboard

6 – Mapp the domain under the Tools – Domain Mapping which you can again find it in the Network Dashboard

Reference URL – http://fronterahouse.com/blog/ultimate-wordpress-multisite-domain-mapping-tutorial/

Reference Site2 – http://premium.wpmudev.org/manuals/wpmu-manual-2/creating-a-network-to-enable-wordpress-multisite/