Download the JDBC driver and install it
The datasource files (pacs-mssql-ds.xml and arr-mssql-ds.xml) of the MS SQL DCM4CHEE distribution contain references to the jTDS JDBC driver.
To use the default configuration, download this driver from the jTDS website, and copy the driver JAR file to server/default/lib. You may also use the Microsoft JDBC driver, but you'll have to modify the datasource files directly to reflect the correct driver name and JDBC URL. Using the Microsoft distributed driver may have some benefit, based on your usage, because it provides support for mirrored database deployments. Do your research and make your own decision, but the jTDS driver will get you up and running quickly with the least hassle.
After installing the JDBC driver, and making any necessary modifications to your datasource files, you should be good to go.