Database diagram support objects cannot be installed

Simply try this.

Alter AUTHORIZATION ON DATABASE::YourDbName TO [sa];

This worked for me

ALTER AUTHORIZATION ON DATABASE ::DATABASE TO sa

Have you tried this set of steps, it might solve your problem:

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 Dropdown at right labeled "Compatibility Level" choose "SQL Server 2005(90)"
4. Go to the Files Page
5. Enter "sa" in the owner textbox.
6. Hit OK

Assuming this works and you can add DB diagrams, you can then change the owner back to your Windows user name.