here's a tip for you or anyone else looking to put bootcamp on, I wish i'd seen this when I did it all:
When bootcamp 'makes' the windows install on your flashdrive within the bootcamp osx tool - let it do it's thing, then when it starts downloading the bootcamp drivers, cancel it.
That download is notoriously slow, probably for a reason knowing apple... I have a 100meg connection and it was still hanging a few hours later. Unless they've changed it since yosemite, cancel the download and google the bootcamp drivers you need for your exact mac. There are different ones for each year/configuration of imac.
EDIT: this is the one you need
http://support.apple.com/kb/DL1721
Once the drivers are downloaded, put them in the root folder of your flash drive (which will have other stuff in it now from the bootcamp tool) - then restart your machine, hold ALT at statup and choose your windows flash drive to boot from, the installer should start from there.