See the round thing next to the hygrometer with the grate in front of it? That comes off and it should have a sponge inside. Thats where the distilled water goes. Or you could use 50/50 PG/distilled water solution, they sell it at most tobacco shops and all online cigar shops have it as well, and it will help regulate the humidity better.

There are also other options for humidifying other than the sponge your humidor came with, look into those as well.

Also do a quick search on seasoning your humidifier before you load it up with smokes