It's not clear from the readme how the textures work - looking at the promos they are probably simple replacements for the basic textures, in which case you should be able to use them, but you are going to have to edit the paths if you don't leave everything in the default location since the textures appear to be in the material library folder (on the other hand, since you can't see that folder in the Content palette it won't get in the way if you leave it, and the jpgs, in the default location). Remember that even if you convert the mc6 files you aren't going to get anything done in the ShaderTree section (anything done with nodes in the Advanced version of the Material Room), though if you can work out what's missing you could rebuild it in the Shader Mixer if you have DS3 Advanced: I notice that there appear to be no bump or specular maps, so those may be things that used shader nodes (though the sample renders look very shiny so perhaps there are no bump settings).