From the readme:

Instructions
------------
1. Copy the contents of MS_ROOT into the root of your memory stick.
(This will overwrite the first GripShift savegame slot).
2. Launch the US version of GripShift.
3. Load up the game (if it doesn't autoload).
4. See your PSP run unsigned code. :)

It'll autoexit after some time. You can use the home button to exit too if
you've seen enough.