Assuming you'd like to build CHDK for yourself, do the following:
create a file named localbuildconf.inc in the root of CHDK source, add
OPT_FI2=1
to the file.
Then copy platform/fi2.inc.txt to platform/fi2.inc, and fill in the keys for your camera as explained on
this page
# ixus300_sd4000, sx210
ifeq ($(KEYSYS), d4a)
FI2KEY=
FI2IV =
endif
To find the keys, use a hex editor to open the sx210 firmware dump (PRIMARY.BIN), and navigate to the offsets you see on the above link.
After you have done all this, just compile CHDK again, and you'll get the .FI2 in addition to diskboot.bin.
Btw. the sx210 keyboard fixes are now in the official source.