Hey man, did you put that fertilizer in with your medium already? I'm hoping not because that's going to be too hot of a mix for seedlings. There was already low amounts of nutrients in the soil you bought. Seedlings do not need any nutrients for the first 2 weeks, they store enough nutrients to get through that long. The potting soil you bought would be enough alone to to support this plant for almost the first month. You won't need any worm castings yet either.
As for when you plant your seed.. plant it as soon as the seed cracks and is showing the tip of the tap root. Don't wait for it to get longer as you will then create more chances of damaging the root. Place it about 3/4" in the dirt with the tap root facing up. The tap root will grow upwards slightly and then curl back down into the ground giving the head of the plant enough stability to curl push itself up out of the dirt.
As for when you plant your seed.. plant it as soon as the seed cracks and is showing the tip of the tap root. Don't wait for it to get longer as you will then create more chances of damaging the root. Place it about 3/4" in the dirt with the tap root facing up. The tap root will grow upwards slightly and then curl back down into the ground giving the head of the plant enough stability to curl push itself up out of the dirt.