i have download the file a2100-100a-1.0.0-full-stable.zip but in the zip file there isn't the file PS.FI2 needed for update the firmware in my camera.
Well, technically loading CHDK on your camera does not update the firmware as CHDK makes no permanent changes to your camera. But you can use the camera's firmware update function to load a CHDK ps.fi2 file - it just goes into the cameras RAM memory and does not change the NVRAM.
However, the beta versions for the A2100 that I posted do not include ps.fi2 files. You need to load CHDK using the SD card lock booting method.