The instructions aren't quite right actually.. all you need to do for this phone is just check the box "use per default (check to use custom attributes)"...
Then click the option at the very bottom of the list "view the different devices"
Now scroll down more (as it has added more options below) click the second to last option "/dev/block/vold/179:33 (fs : vfat).
Once you have done this go back to the main screen and click the top button "external > internal" you may have to click it more than once for some reason. You can also check the box
at boot" to make it automatically apply at boot. But I think you have to click the above button again after checking the box. Again you may have to do it more than once to make it stick.
I've had it not stick the first time, so you may have to try more than once, and at boot it seems to apply late in the boot process, so just be patient.
---In case they ever alter the app defaults you might have to set the internal and external mounting points as well, but the defaults for the S3 are the same for this phone, and that is what it uses by default in the app. Internal = /mnt/sdcard and External = /mnt/extSdCard (with caps)---