RWY INCURSION WHILE TAXIING OUT FOR TKOF.

Narrative

CAPT WAS FLYING HIS FIRST TRIP OUT OF MIAMI. I HAD BEEN IN MIAMI APPROX 3 MONTHS. CAPT GOT CLRNC FROM GND CTL TO TAXI TO RWY 27L. I DIDN'T HEAR TAXI INSTRUCTIONS AS I WAS BUSY WITH COMPANY ON THE OTHER FREQ FIGURING WT AND BAL. CAPT TAXIED S ON TXWY Y. AS HE APCHED RWY 30; I ASKED HIM IF WE WERE 'CLRED TO CROSS.' HE SAID WE WERE AND TAXIED ACROSS. RWYS IN USE WERE RWY 27L; 27R AND RWY 39. AFTER WE CROSSED RWY 30; THE GND CTLR ASKED US WHERE WE WERE. CAPT REPLIED 'JUST CROSSED RWY 30.' THE CTLR THEN TOLD US TO 'HOLD SHORT OF RWY 30.' CONTRIBUTING FACTORS WERE: 1) CAPT'S UNFAMILIARITY WITH MIAMI ARPT. 2) MY BEING BUSY WITH COMPANY; AND NOT BEING ABLE TO SHOW HIM THE CORRECT TAXI RTE. (I HAD NEVER TAXIED TO RWY 27L BY XING RWY 30 BEFORE; SO IT FELT STRANGE.) 3) MIA HAS THE MOST POORLY LABELED TXWYS AND RWYS OF ANY ACR ARPT I HAVE OPERATED OUT OF. TO CORRECT THE PROB: 1) CAPT SHOULD HAVE HELD THE TAXI OUT UNTIL I WAS COMPLETED WITH MY PAPER WORK SO I COULD ASSIST HIM. 2) SIGNS AND MARKINGS AT MIA COULD BE IMPROVED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR IS A FO FLYING THE ATR-42 AND - 72 FOR A REGIONAL ACR. HE WAS CALLED ON A SPECIAL STUDY. HE WAS UNABLE TO ADD ANYTHING TO HIS STATEMENT.