PREFLIGHT WEATHER BRIEFING SHOWED CLOSEST REPORTING STATION TO DESTINATION BELOW DEST APCH MINIMUM OF I MILE VISIBILITY BUT FCST TO IMPROVE TO 1/2 MI BY ARRIVAL. AREA FREEZING LEVEL FCST AT 3-4000 FT. FLT TO DEST AT 6000 FT WAS ON TOP OF CLOUDS AND UNEVENTFUL. PLT CONTACTED DEST ON UNICOM AND WAS ADVISED OF 'LOW GROUND FOG AND VERY POOR VISIBILITY.' PLT MADE NO FURTHER EFFORT TO CLARIFY CONDITIONS, COMMENCED NDB APPROACH AND ENTERED CLOUDS AT 4000 FT. ACFT STRUCTURAL ICING OCCURRED DURING APPROACH. PLT SIGHTED RUNWAY LIGHTS BUT LOST CONTACT IN TURN TO ADJUST COURSE & EXECUTED A MISSED APPROACH. DUE TO STRUCTURAL ICING, PLT UNABLE TO CLIMB ABOVE 3800 FT, THEN UNABLE TO MAINTAIN ALT AS ICE CONTINUED TO ACCRETE. PLT WAS UNABLE TO ARREST DESCENT BUT ON VISUAL CONTACT WITH GROUND MANUVERED TO A FORCED LANDING IN AN OPEN FIELD. PLT REPORTED CONDITIONS AT DESTINATION WERE ABOVE APPROACH MINIMUMS.