The remote MS SQL is activated by default. You need to create database, and database user and password through your hosting control panel.
To connect to the database you need to use the server address listed in "Database server" field when you were creating your database in Plesk and the database user and password you setup in Plesk.
Please note that to connect to MS SQL 2008 Server you need Microsoft® SQL Server® 2008 Management Studio Express. You can download it free from: http://www.microsoft.com/downloads/details.aspx?FamilyID=08e52ac2-1d62-45f6-9a4a-4b76a8564a2b&displayLang=en