Database diagrams Problems

Diagrams problems in sql2005
Error:
------------------------------
Database digoal support objects cannot be installed because this
Database does not have a valid owner. To continue, first use the files
Page of the database Properties dialog box or the alter authorization
Statement to set the database owner to a valid login, then add
Database digoal support objects.

Solution:
1. Right-click the database and select "properties"
2. Click "options" on the left"
3. Select 'SQL Server 2005 (90)' in 'compatibility level )'
4. Click "Files" on the left"
5. Enter 'sa 'in the owner'
6. OK