A C501 CAPT BECAME CONFUSED WITH HIS INST SETUP AND DSNDED BELOW A MANDATORY XING ALT. ERROR ADMITTED.

Narrative

I WAS CHKING OUT A NEW COPLT ON THIS TRIP AND INSTRUCTING HIM ON SETTING UP THE NAVAIDS AND LORAN. WE WERE USING SEA IN NAV #2 WHILE BEING VECTORED TO THE LOC BACK COURSE FINAL FOR RWY 31 BFI. I SET NAV #1 FOR THE ILS FREQ BUT DID NOT SWITCH THE DME DISPLAY FROM NAV #2 TO NAV #1. DURING THE PERIOD THAT I WAS SETTING UP NAV #1 AND IDENTING IT; I WAS INTERRUPTED FROM MY NORMAL FLOW BY INSTRUCTING THE COPLT ON HOW TO ENTER AN INTXN INTO THE TRIMBLE (LORAN). I WAS FURTHER DISTR WHEN I COULDN'T FIGURE OUT WHY THE INTXN WAS NOT COMING UP ON THE ARGUS DISPLAY. I DECIDED TO FORGET ABOUT THE DISTR AND JUST FLY THE LOC BACK COURSE USING DME. BUT; WITH THE WRONG DME BEING READ OUT ON THE HSI; I DSNDED FROM 2600 FT TO 1600 FT TOO EARLY. APCH CALLED BFI TWR WHO RELAYED TO ME A LOW ALT ALERT. I CLBED WHILE FIGURING OUT AND SETTING IN THE CORRECT DME DISPLAY AND; AT THIS POINT; I WAS PAST THE 2600 FT RESTR (LACRE) AND CLRED TO DSND TO 1600 FT. APCH SAVED MY LIFE! WHILE THERE WERE DISTRS; THEY WERE MINOR. I JUST NEED TO BE MORE DILIGENT IN CHKING THE NAVAID SETUP. I NEED TO DOUBLECHK IT JUST LIKE I CHK TWICE FOR 3 GREEN. I WAS ALSO DIVIDING MY ATTN BTWN INSTS AND CHKING THE BUILDUP UNDER THE HEATED LEADING EDGE OF THE WING.