How do you getvyour sim to use the herbicide bench?

Once you've purchased the herbicide bench (from the garden section of the home store) and placed it on your property, you must go to the neighbourhood view and enter the supermarket. Once there you can purchase the herbicides from the fruit and vegetable section.

When you have the Items, go back to your property, any property where you have a herbicide bench (and presumably veggie patches with carnivorous plants in them), and you can click on the carnivorous plant and use the purchased herbicides on them to kill them.

Clicking on the herbicide bench itself doesn't display any options, but your sim will use it automatically when you choose an action against a carnivorous plant.