NON ADHERENCE TO ATC INSTRUCTION. CLRNC INTERP. WRONG APCH USED INTO MCG.

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WHILE ENRTE TO MCG VIA V-480 CTR INSTRUCTED US TO MAINTAIN 8000 FT UNTIL MEFRA (22 DME) THEN DSND AND MAINTAIN 4000 FT. CONSIDERING THE PREVIOUS WX I HAD RECEIVED; I ANTICIPATED VFR CONDITIONS DURING THE DSCNT PAST MEFRA. CONSEQUENTLY; I WAS NOT AS MENTALLY PREPARED FOR THE UPCOMING INST APCH AS I WOULD HAVE BEEN OTHERWISE. APCHING MEFRA WE RECEIVED NEW WX. IT HAD DETERIORATED. TAKEN SOMEWHAT OFF GUARD; I SHIFTED MENTAL GEARS AND DISCUSSED THE APCH OPTIONS WITH MY CAPT. WE DECIDED THE VOR DME C APCH WOULD BE BEST UNDER THE CONDITIONS. MY CAPT BRIEFED THE APCH THEN HANDED IT TO ME FOR REVIEW. OFTEN IN LOW DENSITY TFC AREAS; SUCH AS MCG; CTR WILL ASK 'WHICH APCH ARE YOU REQUESTING' RATHER THAN CLR YOU FOR AN APCH OF THEIR CHOOSING. EXPECTING THIS QUESTION FROM CTR FOLLOWED BY MY PNF CAPT'S RESPONSE; I PREPARED FOR THE VOR DME C APCH. I DSNDED PAST MEFRA AND PREPARED TO JOIN THE 10 DME ARC; WHILE HEARING THE APCH CLRNC EXCHANGE TAKE PLACE BTWN THE CAPT AND CTLR. NEARING THE ARC I GLANCED AT THE HDG OF THE APCH PLATE AS A HABIT TO ASSURE MYSELF THAT I WAS FLYING THE APCH DISCUSSED. IT WAS AT THAT TIME I BRIEFLY QUESTIONED MYSELF IF I HAD HEARD VOR DME C IN THE CLRNC FROM CTR. NOW JOINING THE ARC I DISREGARDED THIS THOUGHT AND MADE SAFE AND PROPER EXECUTION FF THE APCH A PRIORITY. SOON AFTER ESTABLISHED ON THE ARC; CTR HANDED US OFF TO MCGRATH RADIO. I THEN HEARD THE CAPT ANNOUNCE THAT WE HAD BEEN CLRED FOR THE VOR DME C APCH. MCGRATH RADIO REPLIED WITH THE STANDARD WIND AND TFC INFO. THIS REINFORCED MY MIND SET THAT EVERYTHING WAS GOING AS IT SHOULD. HOWEVER; AFTER INTERCEPTING THE FINAL APCH COURSE MCGRATH RADIO BEGAN TO QUESTION OUR LOCATION AND WE SOON LEARNED THAT WE HAD BEEN CLRED FOR THE VOR DME 16 APCH INSTEAD OF THE VOR DME C APCH. THE CAPT OFFERED TO GO 'MISSED;' HOWEVER; WE WERE TOLD THAT WOULD NOT BE NECESSARY. WE DSNDED PAST THE FINAL APCH FIX; BROKE OUT; AND LANDED SAFELY. A SERIES OF EXPECTED SITS THAT NEVER TOOK PLACE LED TO THE MISCOM. I EXPECTED GOOD VFR WITH NO NEED FOR AN INST APCH. UPON RECOGNIZING THE NEED FOR AN APCH I EXPECTED CTR TO EITHER TELL WHICH APCH TO EXPECT; OR ASK WHICH APCH WE WOULD LIKE. THE CAPT AND MYSELF PLANNED TO EXECUTE THE VOR DME C APCH. SUPPLEMENTAL INFO FROM ACN 274867: THE LAST WX I RECEIVED WAS VFR SO I WAS NOT CONCERNED WITH NEEDING LOW MINIMUMS. I SELECTED THE VOR DME C APCH. THIS WOULD ALLOW US AN ADDITIONAL 5 MI TO REACH 4000 FT AND STABILIZE BEFORE TURNING ONTO AN ARC -- AS COMPARED TO THE OTHER AVAILABLE APCHS. MY PAST EXPERIENCE INTO MCGRATH ALSO INDICATED THAT WX TO THE SE OF THE ARPT WAS USUALLY BETTER THAN WX TO THE NW. I TOLD MY COPLT OF MY RECOMMENDATION AND HE SEEMED TO AGREE WITHOUT QUESTION AND BRIEFED THE APCH. AT MEFRA WE BEGAN OUR DSCNT TO 4000 FT AND I ADVISED CTR. THEY ACKNOWLEDGED BY CLRING US FOR THE VOR DME. (AT THIS POINT MY MIND HEARD 'C;' THAT'S WHAT WE HAD DISCUSSED IN THE COCKPIT AND THAT'S WHAT WE WERE PLANNING.) I THOUGHT I ACKNOWLEDGED BY SAYING 'CLRED FOR THE VOR DME 16 APCH; GOING OVER TO RADIO.' I THINK THE PROB WOULD NOT HAVE OCCURRED IF CTR HAD TOLD US WHAT APCH TO EXPECT WHEN HE GAVE US THE ALT RESTR AT 50 DME. WE WOULD HAVE PLANNED ACCORDINGLY.