I just did this and it worked, so give it a try.
1. Do another low-level format of the card with the camera (I checked and it is FAT32).
2. Transfer the files from this link to the card root directory:
download the full package for:
-> sx260_100b
-> sx260_100c
3. You should see the file "
ver.req" not "vers.req" in this download. Only "ver.req" works with the sx260.
4. Leave the card UNlocked and insert it into the camera.
5. Press the PLAY button and let it start.
6. While holding the SET button down, press DISP to see your version number. Pressing DISP again shows another page (contains the last error number in my case)
7. Now press MENU and scroll to FIRMWARE UPDATE
8. CHDK starts. Press the PLAY button briefly to enter ALT mode.
9. Press MENU and scroll (up) to Miscellaneous Stuff / SD Card / Make Bootable
10 Now you can remove the card, lock it, and re-insert. CHDK will load whenever the camera turns on with the card locked.
11. The PLAY button is used for ALT. Hold the PLAY button a little longer for the original PLAY button.
12. Have a beer.