In the main menu you generally have to press the F1 key to activate it, but that's (luckily enough) not the case with the Generals trainers.
Launch a solo game (trainers won't work in multiplayer, doh), and press the desired keycombination for the trainer's option.
The .zip or .rar generall has a .nfo or a .txt file that explains to you what keys are bound to what options.
[Edited by nick1699, 3/15/2010 4:58:59 AM]
If you couldn't use them yet, post here that i help you.
It means your desired trainer doesn't have any.
Therefore, Launch the trainer before running game and while playing the game press the following keys according to available options.
It's so easy!
Well, the result?