After you install BuffBar plugin, go to the lotro plugin manager and select BuffBar. Once you are in, select the QUICKSLOT tab and select the Poison in the window below. Click the EDIT button after that.
You should see the pic below
After you click the EDIT button, you should see the windonw above. Simply open your bags and drag your poison pot into the QUICKSLOT.
The BUFFBAR quickslot ( you need to check where the default position is, move it where it will catch your eye) will pop up when you have a potable poison and you just click it.
You can do the same for the other triggers in the original window.
Don't forget to click ACCEPT in the TRIGGER window as well as on the QUICKSLOT tab when you are done to save your changes otherwise you end up having to do ths every time you log in.
Enjoy