Most textures come with MAT files. You apply the textures using the MAT files. (In the "Pose" section of your library.)

If you have V3's high-res textures, you can use the P5 MAT files from the link I posted above. If you're using other textures, you may have to use the PP MAT files, and then do some adjustments in the Material Room to get the best from the textures in Poser 5.