I am now booting OSX from an eksternal usbdrive. Most things work ok, but not sound (posted in another thread)

I want to install drivers for my Pixma printer, I have downloaded the dmg, and try to run the pkg-file. It won't show my usbdrive as an alternative... Only the two native partitions on the ATV...

Any idea how to make the pkg file recognize my external usbdrive?