The error message (why didn't you simply quote it here?) is:
Warning: mysql_connect(): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2) in [snip]/system/databases/mysql.class.php on line 108
Cannnot connect to DB server
So, either MySQL isn't running on your server or the $_DB_host name is not correct.
bye, Dirk