The Conference Board reported that its Help-Wanted Advertising Index fell 2 points from the previous month. The index measures the volume of classified job advertising in 51 major newspapers across the United States. It is sensitive to changes in the demand for labor as well as general business conditions. The seasonally adjusted index declined to 142 in January from 144 in December and 143 in January, 1986, the business research group said. It reported that help-wanted advertising volume increased in the Pacific region and the East South Central area.