He can log into the panel, and then go to Users > Account Privileges and then “Grant Privileges to a new person”. There he can grant you access to the appropriate database. He will specify your email address on this page. If you already have a dreamhost account using that email address for your panel login then just log into your panel. If you don’t have a dreamhost panel account and don’t receive an email telling you what to do then just try the reset password for panel.dreamhost.com using your email address to get access.
What kind of account does he have? Shared hosting? VPS? Dedicated server? Dreamcompute? the answer to that question will help answer your questions about root FTP access and chmod permissions. Likely it is shared hosting and if that is the case there is no root access whatsoever, however he could likely give you shell access that would allow you to use chmod and other shell commands.