Database Diagram Support Objects cannot be Installed … no valid owner

In SQL Server Management Studio do the following:

1. Right Click on your database, choose properties
2. Go to the Options Page
3. In the Drop down at right labeled “Compatibility Level” choose “SQL Server 2005(90)”
   3-1. choose “SQL Server 2008” if you receive a comparability error.
4. Go to the Files Page
5. Enter “sa” in the owner textbox.
   5-1 or click on the ellipses(…) and choose a rightful owner.
6. Hit OK

after doing this, You will now be able to access the Database Diagrams.