You most likely have to go into the aircraft.cfg file and change the line that reads atc_airline=Boeing to atc_airline=Continental for that particular plane - here is the part you'll need to change in your .cfg file:

[fltsim.2]
title=Captain Sim 757-300 FSX 2
sim=cs757-300RR
model=RRW
panel=
sound=RR
texture=continental
kb_checklists=
kb_reference=
atc_id=N75857
atc_airline=Continental
atc_flight_number=757
atc_heavy=0
atc_parking_types=
ui_manufacturer=Boeing
ui_typerole="Commercial Airliner"
ui_createdby="Captain Sim"
ui_type=757-300 RR
ui_variation=Continental
description=Block 300 - 757-300 Expansion Model v.4.1 FSX \n(c) 2008 Captain Sim \nwww.captainsim.com

Good luck

Jim