AFTER TAKING A WRONG TURN ONTO A TXWY; AN MD88 FLT CREW TAXIES THE ACFT'S NOSEWHEEL OFF THE TAXI SURFACE WHILE UNDER THE GUIDANCE OF A MARSHALLER AT PIT; PA.

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ON APCH BRIEFING FOR RWY 32; WE DISCUSSED HOW WE EXPECTED TO EXIT THE RWY AND TAXI TO GATE. ON LNDG ROLLOUT AT APPROX 40 KTS; TWR GAVE US TAXI INSTRUCTIONS COMPLETELY DIFFERENT THAN WE EXPECTED; EXITING THE RWY TO THE L INSTEAD OF THE R. UPON EXITING THE RWY; WE MISSED THE SHARP L TURN ONTO TXWY P AND TAXIED A SHORT DISTANCE DOWN N5 WHICH DEAD-ENDS. WHEN WE REALIZED OUR ERROR; GND CTL TOLD US TO WAIT WHILE COUNTY AND COMPANY 'FLAGMEN' WERE CALLED TO ASSIST IN GETTING US TURNED AROUND. AFTER THE APPROPRIATE PERSONNEL SHOWED UP; WE WERE INSTRUCTED TO TAXI DOWN TO THE TURNAROUND POINT AND FOLLOW THE MARSHALLER'S SIGNALS. AS WE SWUNG THE NOSE THROUGH APPROX 170 DEG OF THE TURN; THE MARSHALLER CONTINUED TO WAVE US ON THROUGH; EVEN AS WE CAME TO A STOP WITH THE NOSEWHEEL OFF THE TAXI SURFACE. A TUG WAS CALLED OUT; A TOWBAR CONNECTED; AND THE NOSEWHEEL BROUGHT BACK ONTO THE TXWY. THERE ARE SEVERAL THINGS THAT COULD BE DONE TO PREVENT THIS FROM HAPPENING AGAIN. ONE WOULD BE TO BRING THE ACFT TO A FULL STOP WHEN EXITING THE RWY UNTIL IT CAN BE DETERMINED EXACTLY WHERE WE ARE. ANOTHER SUGGESTION WOULD BE TO HAVE A SIGN WHERE N5 MEETS TXWY P STATING 'ACR X HANGAR.' GND CTL INFORMED US THAT MISSING THIS TURN IS NOT AN UNUSUAL OCCURRENCE; AND A SIGN MAY GIVE AN ADDED CLUE THAT N5 IS NOT A THROUGH TXWY. LASTLY; GND PERSONNEL SHOULD PROBABLY BE BETTER EDUCATED TO THE FACT THAT A 130000 LB AIRLINER CANNOT ROLL THROUGH THE GRASS LIKE A TOYOTA. TALKING TO THE MARSHALLER AFTER THE INCIDENT; I WAS LEFT WITH THE IMPRESSION THAT HE WAS FULLY AWARE OUR NOSEWHEEL WAS GOING TO DEPART THE PAVEMENT; BUT THE WHEELS ONLY HAD TO ROLL THROUGH A FEW FEET OF GRASS BEFORE THEY WOULD BE BACK ON THE TXWY.