The you need to apply them through the Surfaces pane, in the Editor tab, to the properties they control - click the little button next to the colour/strength slider and then use Browse to load the map. By default a newly imported mesh will use the Daz Default Shader - for a purely Iray material you will probably want to apply the Iray Uber Base shader, which you can do by having the model and the Surfaces selected and then clicking the preset in the Presets tab of Surfaces, under Shaders>Iray