I haven't had any problems applying the poisons...at least not with Tristella. But she has the
Use Poison feat, so there's no chance that she'll fail to apply the poison. Just right-click on the poison vial, Use it, left-click on the weapon you wish to poison and you're all set.
Of course, for anyone who doesn't have the Use Poison feat, there's a chance of failure when you go to apply it; if you fail, you'll lose the vial of poison and there's a chance that you may even poison yourself with it. But if you succeed, just Examine the weapon. Since this server uses poisons that aren't standard with Neverwinter Nights (yet
are standard with D&D 3), the description will have a blurb in blue text reading something like "Poisoned with Level 5 poison (Temporary)". Or something like that.
That reminds me of a text bug with the poisoning process. I'll have to make another thread about that in the Problems forum....
But anyway, I haven't really had any problems appying poisons. The only problem I've had is figuring out the right situations for using it. Buy a bunch of easy-to-resist poisons (like Nightshade, surprisingly, despite the plant's horrible reputation) and you're just wasting your pocket change; enemies will shrug them right off. I think I've only had two enemies fail their saves against Oil of Taggit (which Tristella keeps around to help her subdue and capture baddies), and since that poison doesn't put an enemy to sleep until the second failed Fortitude Save, both of those enemies had already been subdued from all the whip-flogging by the time the Oil of Taggit kicked in anyway. So I'm starting to wonder if it's just taking up inventory space. :-/
But buy a really expensive and really surefire poison like Dragon Bile and you'll never use it, simply out of fear of whiffing your first round of attacks (since the tougher baddies who deserve to get Dragon-Biled usually have the better Armor Classes) and wasting hundreds or thousands of Talons.
But when it works, poison is a pretty nice way to chisel a nasty enemy down to a more manageable size. Just remember: No undead, no constructs, no elementals, no oozes, no plants and no Outsiders. Anything else is probably fair game. And since slave merchants don't reduce your pay for captives whose Abilities have been reduced by poison, feel free to poison merrily away. ;)