To create a Scratchbox target for ARM and select it as your compilation target:
[sbox-PC: ~] > sbox-config --list-compilers
[sbox-PC: ~] > sbox-config --create-target=ARM --compiler-name=[one of the ARM compilers]
[sbox-PC: ~] > sbox-config --select-target=ARM
[sbox-ARM: ~] > sbox-config --copy-clibrary
Then setup CPU transparency to work between Scratchbox and your ARM device. After you've done that, your target should be ready.