AN SMA TRNEE PLT DIALED IN THE WRONG FREQ FOR TWR AND FOULED UP THE TFC PATTERN.

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I WAS FLYING FOR PRACTICE WITH AN INSTRUCTOR; ON THE TTN ILS 6. WE WERE TALKING TO PHL APCH. WE WERE LEVEL; ON THE LOC; AND WERE SUPPOSED TO CIRCLE TO RWY 34. I WAS UNDER A HOOD. THE HDOF FROM APCH TO TTN TWR WAS UNUSUALLY LATE; AND BY THE TIME WE WERE TOLD TO CHANGE FREQS; WE WERE WELL PAST THE OM; DSNDING. WHEN I CHANGED FREQS TO CONTACT TTN TWR; I ACCIDENTALLY TUNED 121.7 INSTEAD OF 120.7. I DIDN'T REALIZE THIS UNTIL MY 2ND OR 3RD CALL AND THEN TUNED THE PROPER FREQ. BY THIS TIME; I HAD FLOWN PAST THE CIRCLE-TO-LAND MINIMUM ALT; AND WAS ABOUT 1/2 MI FROM THE THRESHOLD OF RWY 6. THE TWR CTLR WAS FRANTICALLY CALLING FOR US TO TURN L AND CLB TO AVOID PATTERN TFC; AND I COMPLIED. THE BASIC CAUSE OF THE PROB WAS MY LACK OF CURRENCY (WHICH IS WHY I WAS PRACTICING; OF COURSE). THE 121.7 ON THE RADIO 'LOOKED' RIGHT; BUT OF COURSE; IT'S A COMMON GND CTL FREQ WHICH I SHOULD HAVE REALIZED (MAYBE THAT'S WHY IT LOOKED GOOD?); AND I SHOULD HAVE CHKED AGAINST THE APCH PLATE AFTER THE FIRST FAILURE TO GET A RESPONSE FROM TTN TWR. THE FACT THAT WE GOT THE HDOF TO TTN SO LATE AGGRAVATED THE SIT BY DECREASING THE TIME I HAD AVAILABLE TO CATCH MY FREQ ERROR. ALSO; I SHOULD HAVE FLOWN THE MISSED APCH AT 680 FT INSTEAD OF WAITING SO LONG; ALSO ATTRIBUTABLE TO MY GETTING BEHIND AND THINKING SLOWLY. IF IT HAD BEEN ACTUAL IMC; AND I HAD BROKEN OUT OF CLOUDS BELOW 700 OR SO WITHOUT RECOGNIZING THE FREQ ERROR; I SUPPOSE I SHOULD HAVE CIRCLED TO RWY 34 AND LANDED; BASED ON THE ASSUMPTION THAT I HAD LOST COM. ALSO; I HAD BEGUN MY DSCNT DOWN THE GS LATE (I WAS OUT OF PRACTICE AND GETTING A LITTLE BEHIND THE PLANE); AND I PICKED UP SOME EXTRA SPD INTERCEPTING THE GS. THIS ALSO COMPRESSED THE TIME I HAD FOR DECISION MAKING. DESPITE WHAT OCCURRED; I DON'T BELIEVE A DANGEROUS SIT EVER EXISTED; SINCE WE WERE VMC; AND MY INSTRUCTOR COULD SEE ALL TFC. I CAN UNDERSTAND WHY THE TWR CTLR WOULD HAVE HAD A DIFFERENT PERSPECTIVE; OF COURSE.