WordPress MU

after uploading the files and run install-index.php, there's error message below:

No WPMU site defined on this host.

Please let me know how to settle, please don't just say check something ,something...tell me in detail.

http://trac.mu.wordpress.org/wiki/DebuggingWpmu

if( $current_site == null && defined( "WP_INSTALLING" ) == false )
die( "No WPMU site defined on this host." );

Seem that the problem is in the 'DEFINE'.