The scale with red numbers is an EV scale. EV numbers indicate a series of shuterspeed and aperture combinations that all produce the same exposure.
Opposite the scale with red numers is an indicator, a small triangle, pointing at the EV value belonging to the aperture shutterspeed combination that happens to be set on the lens.
Some light meters indicate their results in EV values. You then set the rings of the lens so that the indicator points at the value indicated by the meter.
By pressing the tiny black knob on the aperture ring, to the right of the aperture scale, you can lock aperture and shutterspeed rings. If you do, you can move both while keeping the EV value constant; when set right to begin, ou can use any of the "coupled" settings and get the same exposure.
A very convenient thing, that!
The PME45 does not meter flash. So it's not bothered about whether you do have a flash unit or not; there is nothing you have to, or even can set.
When using TTL-flash flash and a camera with TTL-control, you set any aperture, and any shutterspeed, and the flash will (try to) control exposure automatically, by varying flash duration/output.
You will have to connect the TTL-cord to the socket on the side of the camera, and the synch cord to the PC-terminal on the lens.
When not using TTL-flash control, you still can set any shuterspeed, but must make sure that the aperture set on the lens is the same as set on the automatic flash unit.
There is only one connection then: a synch-cord between flash and PC-terminal on the lens.
The "switch button" you mention appears to be the depth-of-field preview lever, also know as the stop-down lever. You can manually stop down the lens to see approximately how depth of field will be at the aperture the lens is set to.
After having a look, you open the thing again, so you have a bright viewfinder image.
And it must be open too when metering using the PME45. It is important that you program the aperture you are using when metering in to the PME45, and then only use that aperture when metering. If you don't, exposure will be off.
It makes sense to meter with the lens wide open: you get the brightest image, which is not only good for focussing, but can mean the difference between getting a reading or not at all in low light levels. And if you do not choose to meter with thelens fully open, you will have to use the stop-down lever each and every time you want to meter.
