Radiance is included in the OpenStudio installers these days. If you grab OpenStudio and install it, You'll also have Radiance (and EnergyPlus) installed and ready for use by OpenStudio. If you simply want Radiance to install on your own, you can get that here. The "latest version" installers we have posted there are a little long in the tooth, unfortunately; I hope to update those RealSoonNow.