Are you wonder how do you enable the Multi-Site option?
The big integration happens in WordPress 3.0. WPMU’s core has been integrated into WordPress, allowing you to host numerous blogs from a single WordPress installation. Network or Multi-site option is the name given to this new function. This functionality is turned off by default. If you wish to use this feature in WordPress, you must first enable it.
To enable this feature, simply open your wp-config.php file located in your WordPress root folder. Then, wherever in the file, add the following line:
After that, you should be able to access the network page by going to Tools » Network.
You may now operate several blogs from a single installation of WordPress, eliminating the need to install additional copies. You’d only be able to run other blogs as a subdomain by default. You can use a Domain Mapping plugin to use different domains for your blogs.