Hopefully some of this might help get the humidity up.
Do you have part of the screen top covered to keep the humidity in?
A Bromeliad will hold water and the light on top will help create humidity.
I use a 4 dollar compact florescent from a hardware sore that has 5500K as the listed light spectrum.
6500K would be better but my hardware stores all had 5500K as the highest but my plants grow like mad so its not proving to be an issue.
A large water dish will also help with keeping humidity.
Also keeping the substrate moist will help too.
You don't have to have a heat pad as long as your home has an average temp of 68-70 degrees at night. The lamp will add the extra heat during the day. A simple misting in the morning and then evening should be enough to keep your Whites happy.
My average humidity is about 50 percent and after a misting will jump to 70-80 percent.
Its not an exact science and your whites will be happy if its warm and they have some moisture and a place to soak.
This is my Whites enclosure (new pics are the 18x18x24 viv)
Frog Forum - DonLisk's Album: My Whites Free Frog Enclosure