How do I correctly set an association between two objects in the Entity Framework 4 Entitydesigner?

I just ran into this myself & when I Googled it (I hit your question).

I was able to double click on the line (association line) in the designer. Fill in the properties there and I got it working.

I also had to close VS and re-open it to get some of the errors reported to go away...

Can't say this is the correct answer - just fumbling around and seeming to get results.

Something that might be easy to miss is to Generate DataBase from model. From what I understand this has to be run after every change made to the Database .edmx.

This simple mistake happened to me and lead to frustration for a few minutes =)