| | Re: What am I doing wrong with my lightbox
Do you have any of your lights aimed at the sides through the tissue paper? I don't see any, that might help brighten it up a bit. Also your white balance on your camera could be adjusted, that will help quite a bit.
An easy free program to brighten it up is Picassa, here is what I did to your picture in 2 seconds by going to "tuning", clicking on the neutral color picker and then clicking on a part of the white background. Then I added a touch of fill light and a lot of highlight!