If the
seed is very hard, it may need to be scarified. ie, scratched or cut to allow water to penetrate. You can use a piece of sandpaper for this or a knife and just damage the outside of the
seed pod. Don't go thru to the embryo
seed inside, then either
plant, or soak and then
plant as you have done.
It is possible too, that the reason the
seed hasn't germinated yet is because the soil has not reached the minimum necessary temperature to induce germination. So maybe once scarifying, you will need to be patient.