First thing you have to know is that creating reports on OpenERP by attacking the DB straight is a bad practice. You might rather favor one of the many tools available like the Pentaho connector, the Jaspersoft connector, the Google Spreadsheet connector,... but this is not the question.

If you want to retrieve the database password, you should get the openerp-server config file on the server machine, open it, and find the right variable.

For a more complete understanding of how passwords work, I suggest you look at this: What difference is there between the Admin and Master pass?