I've read that the wood of an elderberry plant is poisonous, is a green berry just as poisonous?
All parts of Elderberry are toxic, except the flowers and the flesh of the berries - ripe berries need cooking before use to destroy the cyanide like compound in the seeds they contain. I can't imagine why anyone would want to use the green berries for anything, any more than you'd use unripe blackberries -they're unripe and untasty, but the same rules apply - the seeds are toxic, and I'd imagine the green berries themselves may be somewhat toxic; that's hard to establish because no one eats them anyway, so they've not been tested.
Toxicity of the berry seeds is thought to vary between species of Elder - the one most commonly used for its flowers and berries is Sambucus nigra, but Sambucus nigra canadensis (American elderberry), which also has black berries, can be used too.
More information here http://normsfarms.com/are-elderberry-bushes-poisonous/