That's odd. I just tried the trainer and after activating it and selecting a cheat, it instantly crashed the game. I just bought the game from the Halloween steam sale.
I had to run both the game and the trainer as administrator in order to get it to work properly. (Using Windows 7 64-bit)
-Create the Silent Hill: Homecoming executable "SilentHill.exe" as a shortcut on your desktop. App. Location: (Installed Drive):\Program Files (x86)\Steam\steamapps\common\Silent Hill Homecoming\Bin\SilentHill.exe
-Once it's created, right click on the shortcut and choose properties.
-Click the Advanced Button and check the "Run as administrator" box and click the OK button.
-Click the other OK button.
Repeat with trainer or Right Click the Trainer and choose "Run as administrator" option