Follow any guidelines you have available ie. any fitting instructions written on the wrapper.  Failing that, most rubber or foam based underlay's should always be laid rubber or foam face down to the floor.  It's a common error that people think it will work better the other way up, but unfortunately, doing this will generate friction between the carpet carpet backing and the rubber, thus causing it to prematurely wear out and voiding all warrantees, both for the carpet and underlay.