Mejoberries and seeds can be found by harvesting almost any bush in Ark. They are both random drops, with the seeds being very uncommon to rare. Perhaps the best way to acquire a quantity of these berries is either by growing them in a crop plot or by mass harvesting with an appropriate Dino. These berries can be eaten as a food source, though it takes a lot of berries to keep you going. Eating them also helps keep you hydrated.
Mejoberries can be grown in any crop plot and will occasionally produce a seed. Keep in mind that crop plots require water and a form of fertilizer or nothing will grow and the seed will be eventually lost.
For more information on farming, please see this page: Farming In Ark