To poison a weapon you need a piercing one - they are the only ones who will actually be possible to poison - make it stick to your mouse and click it on the poison. You will get a "you have successfully applied the poison" or a "you failed to apply the poison" message. In case you failed, the poison is simply lost (WAAAAAA). If it worked,
the poison will be applied to your primary weapon space and will go off as soon as you do a piercing attack (other than backstab). The poison attack is then checked against the poison resist value of the opponent (means: it can be resisted). As you poison your weapon you will sit down automatically and you have to stand up by yourself.
Taken from
http://kikyara.tripod.com/basics.html