Welcome to the forum !
The 'Make Bootable' button can be unavailable (greyed out) for 2 reasons, shown above the buttons:
1) card is formatted is FAT32 -> you will see "File System: FAT32"
2) card is already bootable -> you will see "CHDK Autorun: ON('BOOTDISK' at 0x40)"
If case 1, you will need to re-format: click "Format as FAT", wait, check if it now says "File System: FAT".
the "Make Bootable" button should be available, click it; because of the formatting, you will need to
re-install your CHDK files from ixus55_sd450-100d-0.9.9-1004-full.zip
If case 2, your card is ready to boot CHDK
in either case, don't forget to LOCK your card (slide tab away from electrical contacts) before inserting
it into your camera or it will not boot CHDK.
Hope that helps, if it does not, don't hesitate to repost,
PS As usual, fe50 was faster
never mind, maybe still useful ...