MGL is definitely on to the issue, which is how mind and body are related.
1) the correlation between brain states and conscious experience does not explain where the sensations of redness, pain etc come from.
Right. This is the so-called "hard problem" of consciousness. If everything is basically just inert matter, where does experience come from?
2) The thesis of pansychism suggests that if mindstuff were not present then there would be no experience. We would just be zombies without consciousness.
That's close. (I'm not at all sure the notion of such zombies makes any sense.) Since we are obviously not without consciousness, then mindstuff is present. The question is whether mindstuff is present only in animals with brains or whether it is present all the way down.
3) Pan-psychism does not suggest that everything has a mind, just that everything has properties that are the constituents ( ie mindstuff ) of the phenomenal attributes of a conscious mind.
Right again. Panpsychism does not claim that rocks have mind or are conscious. It claims that the elemental constituents of which rocks are composed do have an experiential aspect as well as a physical aspect. In non-living things the elemental constituents in their experiential aspect do not combine to form a higher-level consciousness. In living beings they do, and the more complex the living being is physically, the more complex and rich is that living being's experience of its world.
4) A conscious experience could be presumably equivalent to some state of energy in the brain, a state that is determined ( for the sake of argument ) by neuronal activity. Now this neuronal activity may explain our BEHAVIOUR, but it is far from clear how it explains the phenomenal experience. Either it somehow produces it, or the raw phenomenal components - redness etc. - are already part of the energy and particles that the mental state is reducible to. If it produces the phenomena, you need some explanation of how this is possible. I personally can't see how purely spatio-temporal descriptions could explain the qualitative phenomenal properties. The simplest explanation is that these phenomenal properties are already present in the physical constituents of the experience. If that is the case, then everything has some kind of proto-mental attribute, but it is only in complex objects such as the brain that this mindstuff is realised into a mind.
That is the argument in a nutshell.
Look at it this way. It is a plain fact that each of us is conscious and has experience of his or her world. And it is a plain fact that each of us is physical, observable by others. Why would it seem metaphysically plausible that some pieces of reality lack one or the other of these attributes?