I'm running version 1.2.3 of Minecraft (Windows 7) and I didn't managed to make it work. Here is the process I follow:
1. Launching trainer in Admin mode
2. Launching game in Admin mode
3. Moving around a collecting a couple of items, opening inventory
4. Alt-TAB to trainer
5. Alt-TAB to game and then pressing F1
I don't get the Activated message.
Any idea ?

Thanks,

1.2.3 Still works for 1.2.4 still waiting for an update tho