Using the trainer is relatively straightforward:
When Rockstar updated the game to 1.2.0.43, they tightened the anti-cheat and anti-tamper measures. For a trainer to work, it has to "hook" into the game's memory. The game sees this as a virus or a hack. gta iv trainer 1.2.0.43
You will need a dsound.dll or ScriptHook file that works with 1.2.0.43 (often included with the trainer download). gta iv trainer 1.2.0.43