Very cool, glad to see that woomera getting use!
For the resin, cut the pitch with a little beeswax. Will make it less brittle.
Now, attach another length of dowel to the one you have, and you'll probly have enough flex to get going. For instant gratification purposes, duct tape works well for fletching material.
Remember, the flexibility of the dart causes the front end (the point) to resist moving when you start your throw. This resistance loads energy in the dart, flexing a spring. As you follow through with the stroke, the energy is released and the dart goes a flyin. The bigger the fletching, the more stable but slower the flight. If you balance your dart such that its point of balance is about 1/3 back from the point, you won't need fletching, the dart will stabilize itself. For distance fun throwing, try almost no fletching on a 7 foot 7 oz. dart that is moderately flexy. For hunting distances (less than 25 yards, realistically....unless you are throwing into a herd, which is not so feasible these days) a very heavy dart with big fletching is useful. The atlatl dart never achieves anywhere close to the speed of an arrow- instead you must rely on increased mass to result in power.
Have fun!
Oh! and those woomeras are so long, they really need a long dart to work well. About three times the length of the woomera, or more. But you will find that out
