From the readme/instructions:
Max Stats For Selected Driver:
Open the overview screen of a driver showing his stats.
Press hotkey to maximize them. You may need to enter this screen again to see changes.
Did you use the option that way? Does our trainer version still matches the game's version (current: v1.4.14933)?
As I cannot reproduce your issue with our latest trainer, the latest game version and an older save or new game I suggest you verify the integrity of the game cache via the Steam client (Link) and delete your current trainer, wipe your browser cache/Windows temp folder, reboot your PC, download the latest trainer, run it with admin rights and activate the trainer when in the game's main menu as described.
It takes a couple of seconds though until one hears 'Activated'.