the arena does not need to be kept moist, however the farm needs.
You simply fill the farm bottom with the loamy granulate (the red one) and then insert the tube into it vertically. Onto that you fill the rest of the soil.
When you want to moisten the ground for the ants you fill water into the tube and the red granulate absorbs it.
By wetting the red loamy granulate at the bottom you ensure that the whole farm will be moist enough for your ants. You don't need to do this very often, the ground absorbs lots of water and the farm can stay moist enough for up to two months but repeating every 3-4 weeks brings the best results in my experience, however that may vary.