DURING ARRIVAL, THE PLT WAS CLEARED FOR A LOC/DME RWY 15 APCH TO THE UKIAH ARPT. AT 1307 PDT, HE BEGAN THE APCH AT THE TUCCI INTERSECTION, 15.5 MI NORTH OF THE RWY. AT 1311:04 THE LAST RADAR CONTACT SHOWED THAT THE ACFT WAS ABOUT 5 MI NORTH OF THE ARPT. AT 1313, THE UKIAH FSS PERSONNEL REQUESTED THE ACFT'S POSITION & THE PLT REPORTED THAT HE WAS 5.5 MI ON THE DME (5.5 MI NORTH OF THE ARPT). THERE WERE NO FURTHER COMMUNICATIONS WITH THE ACFT. SUBSEQUENTLY, IT CRASHED ON RISING MOUNTAINOUS TERRAIN ABOUT 4 MI SOUTH OF THE ARPT AT AN ELEVATION OF 200 FT. AN INVESTIGATION REVEALED THAT THE DME WAS SELECTED TO THE #2 NAV RECEIVER WHICH WAS TUNED TO THE UKIAH VOR, LOCATED ABOUT 6 MI SOUTH-SOUTHEAST OF THE ARPT. FOR THE APCH, THE DME SHOULD HAVE BEEN SELECTED ON THE #1 NAV RECEIVER WHICH WAS TUNED TO THE LOC/DME AT THE ARPT. THE 1324 WX AT UKIAH WAS 1000 FT OBSCURED, VISIBILITY 1 1/2 MI WITH LIGHT RAIN & FOG, WIND 130 DEG AT 8 KTS.