It's about time I tried to describe its location properly:
The first thing to say is that it's towards the back of the trees, so expect lots of leaves in front of it. If you look at the centre of the cluster of trees which stick above the main canopy and then look a little bit left and down, you should see a few red and blue feathers, and then make out the macaw.
This photo was taken from the river with full zoom. Our guide pointed it out by photographing it with my camera, then zooming in fully. Once I looked away, I'd lose it again.