Downloaded Ubuntu and transferred it to a usb using Unetbootin for mac. My dell mini 9, when I plug the usb in, comes up with 'missing operating system'. Not sure if I missed a step but it appears I have everything on the USB that is needed. Unless I've added something unnecessary like the actual ubuntu 12.10 iso file?
The problem could be one of a couple of things.
If there is no operating system on your mini's main drive, the bios might not be booting from the USB stick at all, and instead is trying to boot from the main drive. This is only the case if you can boot the system normally without the USB stick. If you have a new empty drive, this is certainly a possibility. Reading the docs suggests that you can enter the boot menu by pressing 0 repeatedly during the POST screen.
The other possibility is that the USB stick is being booted, but there is no MBR on it. The MBR is invisible to the user for the most part, so its lack of inclusion would not be obvious. Your best option here is to try making the USB stick again, while reading the instructions again... it is easy to miss that one little detail sometimes. Also, some USB sticks are strange, so it might be worth trying a different one if you have one handy.
Needs to be adjusted for exact BIOS behavior of Dell Mini 9.
You may need to change boot order, to select the USB device first (ahead of internal (?) SSD).
A better method is to force manual boot, and select the USB device. This is usually more reliable.