Using a 'donkey' to lift supports
A donkey is a simple force multiplier which is very useful in treehouse construction to raise heavy supports short distances without expensive equipment. It will lift anything your rope can withstand, and can snap weak ropes.
Making your own donkey
All you need are a few thick ropes (8mm or more) and a Y-shaped fork from a tree. Cut the fork out of very healthy wood with no knots. The best sort of wood is dense, eg oak, holly, maple or beech. Choose arms of thickness around 5cm (2") and of length 50cm (1½'). Larger branches will be able to lift more but are heavy and awkward to work with at height.
Set up your rope so that a loop is hanging down from another support or branch, as in the diagram. Tie another rope onto the support you want to lift and tie this to one arm of the donkey with a bit of slack. Wrap the rope round the end of the donkey a couple of times and then put this end through the rope loop. Then all you have to do is turn the arms until the support is raised. Try and have some more rope ready so you can secure the arms to a branch or something while you work on the support, otherwise it will try to unwind.