Why does my flash gun have these extra settings?

For the zoom setting, match it with the focal length (in mm) of your lens. Match ISO (or ASA) to that of the film you are currently using. Since you're shooting at manual mode, focus on the subject first, read the distance on the lens, check the distance on the flash and refer to aperture recommended. Set lens to that aperture. I'm not using a Metz but I guess it's the same as with other manual flashes anyway.