Parameter is INDEX= in the material definition Deckbuild script statement, see example below. Refractive index file first line is the number of wavelength points, following lines contain wavelength, real part, imaginary part of the refractive index, example also below.
material name = CdTe EG300 = 1.5 NC300 = 8e+017 NV300 = 2.2e+019 INDEX = nkCdTe.dat