Since 9400m has 16 CUDA cores, newer releases of PhysX system software do not support hardware acceleration on it. In newer releases, NVIDIA demands 256MB of memory on the GPU and 32 CUDA cores.
But if you need it for development, testing or just for trying how it works and proving to yourself you can do it… grab old drivers from NVIDIA to get PhysX running on your machine. I've tried 9.09.0408.
Beware, NVIDIA's Fluid Demo is still abysmally slow so I'm not sure if there's any point. (Although I suspect this has more to do with it rendering 60000 particles than with anything else.)
You'll need to manually uninstall the PhysX drivers that ship with your GPU drivers. If you don't see them in Add/Remove Programs, then install the latest version of PhysX drivers (not your GPU drivers) and then uninstall.