First off, find the humidifier. It should be a plastic box with a grill on it, and green foam inside. Just add your water or solution to the sponge.
Another trick is to mix 50% propylene glycol (PG) with 50% distilled water. You can sometimes find PG at a pharmacy, or have them order it for you. Generally when you buy Humidifier solution at a cigar store, that is what you are paying for. Only you are paying a 500% mark up.
Of course distilled water works ok - it the PG that helps prevent a box that is TOO humid. (which promotes mold)
Some times with new humidors, they take awhile to get to a constant huminity (I am embarassed to day that I have neglected mine, and it is a bit dry right now.) One tip it to take a damp rag and wipe the inside of the wood. Now use just a LITTLE bit. You dont want to make the wood wet, in anyway. Doing so could make things start to warp.
If your humidifying device doesnt seem to be cutting it - make your own. Take a plastic travel soap box. Drill a bunch of holes in the top of the lid. Add green floral foam inside (get the kind for REAL flowers - the kind for FAKE ones doesnt work). Now you can add your water/solution to the foam and have more of it to help your humidor along.
FYI - it can take a bit to get it steady. Check out www.jrcigars.com - I used to go to their forum and they were - for the most part - very helpful. Prices are good there too!