I was having same problem which explained in tha linked page. but this page did not solve my problem. how I solve this issue:
1- download sql management studio express
2- in the tree open security node and right click on "sa" user and enable "sa" login.
after that I easily design my database in diagram.
Cannot add diagram to SQL Server 2005 DB: Database diagram support objects cannot be installed because this database does not have a valid owner.