>I am trying to develop in my pc this basic 3 tier solution out of these links

I agree with what Cetin said.

I'm putting my data access and the logic for it into a single project using the Entity Framework and partial classes.

As to your current problem... my best GUESS is that you have the NorthwindDataSetTableAdapters in a different project than the code that you posted and haven't referenced that project in the one you are calling it from so it can't find it.