Natalia,
Please try installing the sqlsrv extension and enable it in PHP.
MSSQLDatabase.php will use sqlsrv if it is installed, instead of the mssql driver with PHP.
Download it here: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=80e44913-24b4-4113-8807-caae6cf2ca05
Cheers,
Sean