From Port Authority (8 Avenue & 42 Street) to Rockefeller Center (6 Avenue between 47 and 50 Streets)...
You can walk the six blocks uptown on 8 Avenue and make a right turn on 48 Street. Then it's just two blocks over to 6 Avenue--which is officially called Avenue of the Americas, but not by most New Yorkers.
You can also pick up the R or W train from the station beneath Port Authority. One stop uptown will take you to 49 Street & 7 Avenue--which is basically the back door to Rockefeller Center.