Hi PsyDuck,
in the server manager, select ibays and then select modify for the ibay you want to enable write access to, now make sure that you have selected a group other than admin, you don't want to logon as admin, then in User access via file sharing or user ftp select write for the group and read for the group, and for Public access via web or anonymous ftp select public password required, and also make sure in the server manager under remote access where it says FTP access limits set it to public if your trying to access it remotley which i take it you are, and i'm not sure but for FTP user account access you might have to make that public (but not sure)
hope that helps
cheers