Imelda Marcos pleaded not guilty Monday to a $268 million federal racketeering indictment charging that she and her husband, deposed President Ferdinand Marcos, used stolen government funds to buy Manhattan real estate. Mrs. Marcos, 59, entered her plea in U.S. District Court in New York City after waiving a reading of the 79-page indictment that could send her and her husband to prison for the rest of their lives. The Marcoses, five others and a California bank are accused of transferring $103 million in stolen funds into the United States to buy four Manhattan buildings and of defrauding two U.S. banks of $165 million to refinance the properties. Mrs. Marcos was taken immediately to a third-floor briefing room. She was to be fingerprinted and pose for mug shots immediately after the arraignment.