That extension is not available in the default php installations on DreamHost.
Note that, per http://us2.php.net/mssql:
" Requirements for Unix/Linux platforms.
To use the MSSQL extension on Unix/Linux, you first need to build and install the FreeTDS library. Source code and installation instructions are available at the FreeTDS home page: » http://www.freetds.org/
Note: In Windows, the DBLIB from Microsoft is used. Functions that return a column name are based on the dbcolname() function in DBLIB. DBLIB was developed for SQL Server 6.x where the max identifier length is 30. For this reason, the maximum column length is 30 characters. On platforms where FreeTDS is used (Linux), this is not a problem.
The MSSQL extension is enabled by adding extension=php_mssql.dll to php.ini.
To get these functions to work, you have to compile PHP with --with-mssql[=DIR], where DIR is the FreeTDS install prefix. And FreeTDS should be compiled using --enable-msdblib."
To accomplish this on your DreamHost server, you will need to compile your own PHP (general instructions are in the DH Wiki article on installing PHP5) in accordance with the instructions above including the installation of the FreeTDS library compiled using --enable-msdblib.
When that is done properly, you can then enable mssql.dll in your php.ini.