The Shader Mixer is the main way to build new shaders, it isn't an exact match for the Poser materials Room but has broadly similar features for 3Delight (Iray is rather moer different) - one thing to note is that in Poser's Material Room an input plugged into a block multiplies with the local value, in Shader Mixer it replaces any local value.

For reflections, just set the Reflection Strength to a non-zero value and have no map assigned to Reflection Colour for the 3Delight shaders; for iray you want non-zero glossiness (Iray does only raytraced reflections)