Hi Lisa,
you are right you have to increase the intensity of these HDRIs.
To use HDRI to light a scene you have to use the Radiosity Tag which gathers the lightning information from the HDRI. See the pig scene.
But I also plan to add a post process called tone mapping to Cheetah3D. It automatically convert the HDRI colors into the [0,1] range a monitor can display. That will make the usage of very dark or very bight HDRI's much easier.
I also plan to allow Cheetah3D to save directly to .hdr or OpenEXR files. This will allow the usage of external tone mapping software. It will also allow to render your own syntetic HDRI's.
By,
Martin