The easiest I can tell you would be to put the create_hdd_region.sh file in the root of either a usb card reader or portable hard drive, and insert it into the first usb port on your PS3. You would then want to change into the directory in which your usb device is mounted: by typing: cd /tmp/petitboot/mnt/sdd1/ ; then you would need to copy that file to root by typing cp create_hdd_region.sh / and go back to root by typing cd / and then chmod 777 create_hdd_region.sh and lastly execute it by doing sh ./create_hdd_region.sh.
However be warned that once you power off your PS3 all the files you've copied over such as create_hdd_region.sh will be removed, but this won't matter in the end. One final important step to do is refer to Step 2 again in the guide, which reads, "When installation is finished, go to recovery menu and choose 'Restore PS3 System'".
Otherwise your PS3 will not see the space reserved for linux. Also the part I underlined sdd1, if you put the usb device in the first port that should be the mount point if not it will be somewhere in the parent directory.