Hold  down  alt  and type 'tac cheat' from the main interface. If
you  mistype or it doesn't work, hold down alt and press any key other
than  't'  then  try again. You will have visual confirmation that the
cheat mode has been activated.

ALT and + - Cancel cheat mode 
ALT + I - Escape Invincibility on/off 
ALT + B - Hidden terrain on/off 
ALT + V - Hidden units on/off