Not all hard shelled seeds need to be nicked, it seems. I don't have much experience with ones that don't, but I've read in germinating guides that a many of them don't necessarily need nicking or soaking. Which is which, I'm afraid you'll have to read up somewhere. If in doubt, it never hurts to nick it a bit just in case.

You don't really need to cover them with plastic, but covering them does help to keep moisture in, as well as keeping the temperature up a wee bit. You will have to remove it (or open it up a bit) once the seeds germinate.