If the Menyoo trainer is not showing up in-game, they will need to install "Script Hook V" and follow steps 6,7, and 8 again.

Those who have installed mods for GTA 5 may have used a tool named Script Hook V for the same at some point.

Installation is fairly simple, you need the latest version of ScriptHookV.dll, Asi Loader (Bundled with Script Hook) and the mod files (.asi).