Sorry to say this but your doing it wrong, your almost there but not quiet.

"Fire the game up, trainer, F1 at main menu,
choose sandbox (Berlin XL 1920), normal difficulty
game is going, see the money up top left, alt tab, right click task bar and click "Show the desktop"
goes to the desktop and i then click the icon for Cities in motion(in the taskbar)"

Your not meant to pause the game, its meant to be running.
By clicking resume or pressing escape your breaking the chain.

"goes back into the game, un-pause the game and then press Numpad 1. Hear the voice saying activated and there's no change in my currency."

Just do as you have done but without pausing, when you click on the game on taskbar after show desktop don't do anything but press Num1, remember it only works if you can see the currency.

[Edited by tabsf1, 3/4/2011 5:32:40 AM]

Reinstalled the game, the trainer worked.
Patched to latest version for retail, trainer doesnt work anymore.
I had to reinstall, hack the money and save then patch again and load the save game. Maybe steam version is not up to date ?

[Edited by HoD, 3/5/2011 1:00:30 AM]

[Edited by wozjtf, 3/4/2011 3:16:12 PM]

Patch 1.0.13 changelog: Link.