Poisons need to be applied, if you have the bow equipped then "use" the poison and the message "do you want to apply to (the arrow you have equipped) (Y/N)?" if yes then the arrow is poisoned and the poison used. Note it only applies to one arrow at a time, misfire that arrow and you've wasted the poison so make it count.
Best enchantment on a bow would have to be paralysis imo, it's ever so useful being able to incapacitate your enemies for 4 seconds or so, they're simply frozen for that length of time. Really handy in melee fights if most of your enemies are lying on the ground motionless.