[Boot Camp] Windows 8 - The installer disc could not be found
I'm running Mountain Lion (10.8.1) on a mid-2010 MacBook Pro and I'm trying to install Windows 8 using Boot Camp. I downloaded the Windows 8 RTM from the Microsoft Volume Licensing Portal. I used Boot Camp for the first time and it gave the option to "Create a Windows 7 USB Disk" which I did using the Windows 8 ISO. I couldn't install Boot Camp at the time, so I decided I'd do it later. I've just re-run the Boot Camp Assistant and I've noticed the "Create a Windows 7 USB Disk" isn't there anymore, and also, when it comes to creating the partition, I get the following error:
The installer disc could not be found.
Insert your Windows installer disc and wait a few seconds for the disc to be recognized.
1) How do I get the option to create a Windows USB Disk again?
2) How can I get the Boot Camp Assistant to recognise the USB I already have?