I don't have too much to offer here but since no one else is chiming in -
Perhaps VS's schema is out of sync with the actual database or maybe pointing to a different db instance?
But being a long term sql server developer I only use Management Studio for this. For instance to create this "relationship" I would use a query window to issue this command:
alter table Paths add constraint FK_Paths_Types foreign key (TypeID) references Types (TypeID)
Can Sql Compact be used in SSMS?
Be One with the Optimizer