I heard that soaking cotton balls with peppermint oil will get rid of mice in your home?

Absolutely tell your landlord! That should be first on your list of things to do to get rid of your unwanted critters. You should not have to tolerate such conditions. And yes, when there's one there usually always two. . .

D-Con is very effective in getting rid of mice. Good you bought it. Never heard about cotton balls and peppermint extract to kill mice. But I have read that the electrical pest control devices are not effective at all. You might want to take it back with your sales receipt and tell them it was completely useless. They'll return your money.

Good luck!

Inside the home or business you need to set traps so you can dispose of the dead critters. Outside, place bait enclosures that only the mice have access to, they will take the bait and croak, possibly in there nests. Bait and enclosures can be purchased on line and the snap traps at a hardware store. Do not wait as having mice around can be dangerous if not deadly, Hantivirus. Peppermint is known to keep ants out of cupboards and cabinets.