I think I forgot to put the associated files for the translucent material in the DIVA-3.0 installer, so thank you for pointing that out! Just save the attached file in c:\DIVA\Radiance\lib\ for now, and your simulations should work using that material.
As for a custom translucency, you would have to make your own custom materials. Pgs 21-23 in Axel's Radiance Cookbook discuss the tricky nature of defining translucent 'trans' material types in Radiance.
Thanks a lot Alstan, that worked :)