Please look at below step for create multi site mode:
- Install WordPress 3.0 and install as a normal WordPress.
- Add define(‘WP_ALLOW_MULTISITE’, true); near the top of wp-config.php
- Log into the admin go to the tools/Network menu.
- If it asks you to deactivate all plug-ins do so otherwise go straight ahead and set up multi sites as sub-domains and fill in the other details as needed as needed.
- Make a note of the changes needed to wp-config.php and .htaccess and make the changes.
- You should now be in multi-site mode.