Ummm, you need to get the x86 32bit version as ammorais said..
Unetbootin must have a iso image to install to USB
you can get debian live cd or usb images, as you know
the i386 versions are what you need.
Debian Live can also run persistent from USB
which I assume is what you want?
that way, at each reboot/startup, your settings, apps added etc are retained
I made a wonderful Debian Live remaster featuring many many apps
and included info on persistent cd/usb/hd install
and also include 2 persistent "files"
1 is 250MB, other 500MB
check it out if you like
I also made the above so you can make your own cut via remastersys and I made it so its easy to install
to USB from iso without using dd.