You will be looking for something the size of a basketball, if it is high in a tree, or of a small dog, if you can see it more closely. Often it is the long pendulous tail that is first sighted in the canopy of a tree.
They are extremely hard to spot. Even the people researching them have a hard time actually seeing them - despite that they have radio collars on their study animals and know fairly exactly where they are.
Looking straight up into the trees is no good! Try looking as horizontal as possible into the canopy, possibly from sloping terrain. Drizzle weather is good tree-kangaroo spotting weather. They come out to the edges or on branches that stick out of the denser foliage to catch the wind-drying-effect - not so in more substantial rain, though.
A good time is late afternoon to early evening, because the females then leave their fixed day spots and start moving around. One can try spotlighting, too - the tree-kangaroo's eyeshine is not as bright as the possum's, but still reasonably stands out from the background in a dull ruby red.
Don't approach the animals too close at night as they are much more skittish than during the day and will readily jump off the trunk/branch and take off.
The photo links above give an indication of what 'extremely hard to spot' and 'looking into the canopy horizontally' means.
The locations shown in the map above give an indication of publicly accessible locations where you have a good chance of seeing a tree-kangaroo in the wild.
and mammal group