Since you are looking for a particular attribute, you need to do the filter before the join, otherwise anyone who has an attribute, but not the one you are looking for will be filtered out. This means that you have to filter in your SQL code. Typically this is accomplished by using a view or stored procedure to do the filtering, but you can also directly edit the SQL for the report. There is one big caveat: once you edit the SQL, opening the visual editor will wipe out any changes that it doesn't understand.
Drew