I'm probably pointing out the obvious to loads of people - but for those only own GC2-EU this is how I got the cheats listed for Dark Avatar to work:
1. Find the Kalypso directory and look for the Dark Avatar subfolder, eg on Vista 64
C:\Program Files (x86)\Kalypso\GalCiv2EU\DarkAvatar\
2. Right click on GC2DarkAvatar.exe and create a shortcut. Then drag the shortcut to your desktop.
3, Right click on the shortcut on your desktop and select properties - in the target window, add the word cheat after a space from the existing target path, eg:
"C:\Program Files (x86)\Kalypso\GalCiv2EU\DarkAvatar\GC2DarkAvatar.exe" cheat
Not the word cheat is OUTSIDE the quotes.
4. Click OK and run using the standard cheats (listed on this site for the "Dark Avatar" version of the game I think). Works a charm!
Again, sorry if this was really obvious to anyone - or if this is an accidental repost.