You do not need to do any of the bootable stuff or partitioning to get the firm update option. Just copy the PS.FIR to the root directory of the SD card.
If you want autoboot, you do need to make the card bootable. However, since the SD630 doesn't support SDHC AFAIK, you should be able to just format the whole card FAT16 (you may have to force this somehow on a 4GB card, since 4 GB fat16 is non-standard.
It sounds like you are using a Mac. Your problem is most likely described in
http://chdk.wikia.com/wiki/FAQ/Mac#Still_Having_Trouble.3F
