Q: Prohibitory Ghost Busters symbol on startup Bootcamp

I just got my new 5K iMac. I did the bootcamp thing. I have a Superdrive with a full version of Windows 8.1 on it. I loaded up a USB drive with the bootcamp drivers. I clicked the last button in the bootcamp set up and created a partition. The iMac reboots, and then a quick screen comes up that says "click any key to boot from drive"

Then the apple shows up, starts loading, and finally I get the prohibitory "you shall not pass" symbol. Nothing happens from this point on, and I have to reboot the computer. I've been through this 3-4 times now.

I am guessing that it isn't finding the superdrive with the Windows disk, right?

How do I get around that? Do I have to have an ISO of the disk even if I have the full version ready to install on a superdrive?

Am I supposed to somehow tell it to boot from the USB stick?

I have been reading around this, but all the posts I have found are old. I've been up since 4 a.m. Forgive me if this is obvious.