Do Kenshin Uesugi's story mode. When you get to the Siege Of Gifu castle level, a mission will appear saying to eliminate Keiji Maeda. Do not do this. After you complete the level, you will appear in Gifu Castle. When you reach the fourth level, Keiji Maeda and Nobunaga Oda will be waiting for you. Then, kill Nobunaga Oda to complete the level. When you finish the Showdown At Kawankajima level and the credits end, a screen should appear stating "You have unlocked Keiji Maeda.