DC10 FLT CREW FAILS TO COMPLY WITH ASSIGNED ROUTING.

Narrative

ASKED BY ATC (TORONTO) TO CONFIRM ROUTING AFTER ECK. THEN DISCOVERED THAT ALTHOUGH ROUTING HAD BEEN SELECTED IN INS; NAV HAD NOT BEEN ENGAGED. ACFT HAD CONTINUED ON HDG RATHER THAN TURING TO NEW COURSE. I HAD NOT FLOWN DC10 FOR NEARLY 90 DAYS. FO WAS FLYING FIRST TRIP AFTER IOE; AND SO HAD BEEN DISPLACED FROM DC9 CAPT. NEARLY CONSTANT QUESTIONS; ANSWERS; AND DISCUSSIONS RELATIVE TO DUTIES; PROCS; ETC; WERE OCCURRING. OTHER INS EQUIPPED ACFT THAT I HAVE FLOWN DID NOT REQUIRE THE ADDITIONAL STEP OF SELECTING NAV WHEN OPERATING ON AUTOPLT AND CLRED DIRECT TO A FIX ENRTE. SIMPLY SELECTING DIRECT TO CLRED FIX WAS ALL THAT WAS REQUIRED. A LACK OF RECENT EXPERIENCE IN THE ACFT BY MYSELF AND AN INEXPERIENCED CREW PLUS NIGHT FLYING CONDITIONS CONTRIBUTED TO THIS OVERSIGHT. THE ACFT HAD 3 MEL'S; WE HAD DIFFICULTY COMMUNICATING WITH GND CREW ON PUSHBACK. THE APU SHUT DOWN WHILE STARTING FIRST ENG LEAVING US IN THE DARK WITH NO PWR. TKOF NUMBERS WERE LATE ARRIVING. THE DEP TURN AFTER TKOF WAS OPPOSITE THAT EXPECTED. WE HAD HAD NUMEROUS DISTRS. THE PRIMARY CAUSE TO THIS NAV ERROR WAS LACK OF RECENT EXPERIENCE BY ME; INEXPERIENCE OF CREW; AND NEED FOR ADDITIONAL STEPS NEEDED TO ENGAGE NAV FUNCTION COMPARED TO SIMILAR ACFT.