A VERY EXPERIENCED PLT RPTED THAT HE WAS ATTEMPTING TO INTERCEPT THE LOC FOR RWY 17 AT OLM; BUT DID NOT REALIZE THE FREQ WAS NOT ENTERED PROPERLY; RESULTING IN A CFIT SIT.

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I WAS UNDER AN IFR CLRNC AND UNDER RADAR VECTORS TO INTERCEPT THE LOC TO RWY 17 AT OLM. I WAS MAINTAINING 2500 FT. I WAS CLRED FOR THE ILS APCH. I HAD SELECTED THE LOC FREQ BUT DID NOT 'FLIP-FLOP' THE FREQ FROM 'STANDBY' TO 'ACTIVE' AS I RESPONDED TO THE INTERCEPT HEADING CALL AND MADE MY TURN. I INTERPRETED MY LOC INDICATOR (NOT TUNED TO THE PROPER FREQ) TO SHOW I WAS INTERCEPTING THE GS AND BEGAN A DSCNT. THE APCH CTLR CALLED ME AND SAID I NEEDED TO MAINTAIN 2500 FT TO THE FINAL FIX AND SAID HE SHOWED ME AT 1600 FT. I IMMEDIATELY NOTED THE FREQ PROB; CLBED TO 2500 FT AND COMPLETED THE APCH. THE PROB WAS MINE: 1) NOT PROPERLY TUNING AND IDENTING THE FREQ. 2) NOT CONFIRMING THE FIX. THE AIRPLANE DOES NOT HAVE DME AND THE MCCORD VOR HAS CONSISTENTLY BEEN OTS FOR YRS. BUT I DID NOT ASK THE CTLR TO RADAR IDENT IT FOR ME. 3) I DID NOT MONITOR THE GS FLAG ON THE INDICATOR. I HAVE NOT FLOWN THIS AIRPLANE OR PERFORMED AS SINGLE PLT FOR 3 MONTHS. I NEED TO STAY CURRENT; AND PAY CLOSER ATTN TO PROCS AND EACH OF THE 3 POINTS ABOVE. I AM GLAD THE CTLR NOTICED THE ALT. THERE WAS NO INTERVENING TERRAIN ALONG THE APCH BUT AT A DIFFERENT ARPT THIS COULD HAVE BEEN A MAJOR PROB.