like tremble, or mother death or the tribal bleak venom, what am i supposed to do? take it? reverse pick pocket? what?

Melee weapons.

Coat your weapons and stab someone with them. Or drink it. Depending on your character's INT.

thats a joke, right? and how do i put it on a weapon?

IIRC you can drink it if you put your character's INT low enough, I haven't done a melee build yet so I don't know exactly how to do it but I imagine it's something like equip the weapon and click on the poison.

my guess would be to pick a melee weapon and press LB if your on xbox or what ever you use to apply mods, since it is basically a mod