Press shift and enter simultaneously to pull up the console, and then type any of the following codes. The names are pretty clever if you know the history behind them.
Advance clock To End Time: Sedgewick (Union); Pickett (Confederate)
Computer vs. Computer: Halleck (either side)
Erase Stress: Hancock (Union); Jackson (Confederate)
Fortify All: Hooker (Union); Longstreet (Confederate)
Rally Troops: Sheridan (Union); Lee (Confederate)
Reinforements: Reynolds (Union); Hill (Confederate)
Reset Clock to Start Time: Buford (Union); Stuart (Confederate)
See all troops: Warren (Union); Hotchkiss (Confederate)
See Enemy Battle Plans: Custer (Union); Harrison (Confederate)