What intake and extraction have you got, is the 6" fan one of them or is that a fan in the tent for air circulation?
What temps are you seeing? That's a lot of light you've got in there

.
You can increase the humidity by putting trays or bowls of water, the more surface area of water the more it'll increase the humidity. You can hang wet towels in there. Or you can get a humidifier, search for Mini-Fogger on the Maplins website which is £15 so not too expensive.
I'd have thought your humidity will start to go up once you put the plants into flower, in my budbox during vegging I don't see above 30% but into flowering and it's up to 50%-60% once the plants are big. How are your plants looking? If they're healthy then I wouldn't worry about it that much, not having high humidity now means you shouldn't have to worry about it getting too high when they're into flower.