mysqlnd is the library that can be used since PHP 5.3, instead of libmysql, by 3 PHP extensions :
mysql, which provides the mysql_* functions,
mysqli, which provides the mysqli_* functons,
- and
pdo_mysql, which allows one to use PDO with a MySQL database.
mysqlnd by itself doesn't export any function you can use from your PHP scripts : it only provides MySQL connectivity to those 3 extensions -- which are the ones that export functions you can use.
If you want to use the mysql_* functions, you have to make sure that the mysql extension is enabled, with something that whould look like this in one of the .ini files parsed by PHP :
extension=mysql.dll
As a sidenote : the mysql_* functions should not be used anymore, especially for new projects : the mysql extension is old, and doesn't allow one to use recent (well, not that recent anymore, actually) features of MySQL.
Instead, you should be using mysqli or PDO.