Resolution:
The name of MS SQL Server driver is sqljdbc.jar or sqljdbc4.jar.  You can use either of them, depending on the BW JRE version and database version in use. For details about system requirements, refer to Microsoft's website: http://msdn.microsoft.com/en-us/library/ms378422.aspx

After the download, you can copy the driver jar file to tibco/tpcl/5.x/jdbc directory. Restart Designer and test the JDBC connection to make sure you are able to connect to the database.