To charge your humidifer, remove it from the humidor and dampen the humidifier in a 50/50 solution of distilled water and propylene glycol (Activator Solution).
Propylene Glycol is a hydroscopic substance that absorbs moisture from its surrounding environment. The distilled water evaporates until the humidity level approaches 70%. The Propylene Glycol then activates and will not allow any more moisture in the air by its absorbing properties.

The ideal is to moisten the humidifier without it becoming soaked. A few squirts of solution will be enough to keep it charged for approximately 60 days or whenever you feel the cigars are starting to become dry at all or when the desired level of humidity has dropped. Place the humidifer in the humidor and wait approximately 1-2 days to gauge the average level of humidity. If this spikes down when placing your cigars in the humidor afterwards, repeat this process as the cigars themselves will be absorbing the moisture within the humidor, occassionally causing a spike downwards in your desired level of humidity.

Do not use tap water under any circumstance. This can only cause harm to your humidor due to the foreign chemicals that can ruin the cedar wood inlay and the general degree of humidity that the humidor can withstand.