Your PHP script calls the mysql_connect() function, that is, it tries to
connect to a Mysql server. Your script assumes that this Mysql server is
on your computer. Since you have no Mysql server running, it can not
connect, that is why you get this error.
You need to contact the author of the script and ask what to do or install
and run a Mysql server on your computer.
mysql_connect() is a PHP function, please see docs on the php.net website.