The setup for SQL Server connections is different from other SQL connections. Users almost always have 'domain' names that accompany their usernames. The standard Windows protocol is to combine the domain and username like this:

domain\username

...and enter the combination where it says 'Username: '. That's not the correct approach when setting up data source connections in MI. The correct approach is to enter the username as-is and to specify the domain by appending it to the jdbc string, as follows:

jdbc:jtds:sqlserver://[servername]:1433/[DBName];domain=[DomainName]