In your select statement there are a couple of potential problems that you need to verify.
select * from datasource.username.table ... which is returning 'NORTHDS.username.Customers'
select * from Northwind.dbo.Customers ... (unless you renamed the database ...)
In other words, check to make sure your are referencing the correct database/owner/table in your select statement. It appears that the variables you are using are incorrect. tablename appears to be ok, but the username variable is returning username. Also, the datasource variable may not be your database name.