haven't played for a couple years and only started again recently but it my memory is correct, when using poisoned weapons and when rolling to hit if you get a 6 then you automatically cause a wound. they unit being attacked still gets armour saves as normal you just don't roll to wound... think it's probably in one of the books
jungle poisons if different to poison attacks as jungle poisons only work with missle weapons and not close combat.
Another thing to note, if you have any modifiers that move your to hit value above 6, you lose those poison attacks. For example, if you move your skirmishers and are still at long range while double tapping (shooting twice) at your target, it is very possible that you will need 7+ to hit your target. Your poison will not work in this case. That is why chameleons rock!