The .hdr is plugged into the color_value of your light node in the Material Room.  Contrast should be set to 1 and intensity reduce according to how the .hdr was prepared.  It is is dark or a night scene then the intensity will be high. On the light end, the intensity will be low.  Remember. the hdr is just providing an ambient light source, there will be no shadows and specularity cast.  Infinite and spot lighting should be introduced separately.  The .jpg image can be used as a background image although it is a panorama of a scene.  If you have HDRShop, those scenes can be converted to lightprobes for Poser 7.  The scenes can also be used in bagginsbill's envsphere although that prop in essence requires equirectangular panoramas.  For single plane use, load the image into a one-sided square, set your scale x and y to the exact pixel dimension of the image and then scale the prop to whatever size you desire for a background. 

I use IBL all the time to give my scenes a more realistic kick.