When you run the simulator for the first time it should ask you exactly that question. You can either give the UUID of the Master avi and then you don't need to give first/last-name and password, otherwise you take UUID 000...(The default) and fill in the name and password. If you want to edit the Regions.ini file manually in the Regions folder then the settings you are looking for are:
MasterAvatarUUID = 609c9e3c-924b-4a2a-82d6-900e0f06f0c1 ;MasterAvatarFirstName = "test" ;MasterAvatarLastName = "user" ;MasterAvatarSandboxPassword = "test"
Notice that in my ini file the First/Last/Password are commented out since I am using the UUID anyway.