ISO button changes the ISO by using the two arrow up and down PR buttons.
F-Max is the maximum aperture you got on the lens you are using
Metering button toggles from Spot [little dot], Integral [square] and Incident light metering [sun with rays]

The red button switches it on.

The most important thing is to make sure your focusing screen matches your finder. The recommended screen for the current PME45s is the Acute Matte D Series screen, but if you use an older screen you can set the finder to compensate. You need to enter programming mode to do this, by pressing the two PR buttons simultaneously.

You could pick up a manual easily, go get one!