Install Windows 7 with Bootcamp on OS X Yosemite with USB Key
Found this on an apple forum. It'll allow you to create a bootable USB on older macs through boot camp without an optical disk. I used this method on my late 2011 mbp and I now have windows 8.1 pro running on it perfectly::

Before you do anything, make a backup of Info.plist or the whole Boot Camp Assitant app so that you can go back if necessary. Rename it something like "Info old.plist" or "Original Boot Camp Assistant."

First, edit the plist of Boot Camp Assistant:


Go to Applications/Utilities
Right click Boot Camp Assistant and view package contents
Find Info.plist in the Contents folder and edit it in Text Edit (it will make you duplicate the file first)
You want to edit FOUR things:

Add your model to DARequiredROMVersions
Delete the word "Pre" from UEFIModels and add your model
Delete the word "Pre" from USBBootSupportedModels and add your model
Remove your model from Win7OnlyModels (if its there)



To add your model to these lines, just copy and paste an exsiting row and replace the data between the two  tags with your model code. You can find the right codes to use for the plist by going to Apple Menu > About This Mac > More Info > System Report. Use the "Boot ROM Version" and "Model Identifier" as appropriate in the plist.


Once you make these changes, save the file and replace the old Info.plist with your new one. It will ask you to type your password.


The last step is to do a code sign. Boot Camp Assitant will not run if it's been edited. You need to resign it. Open Terminal (use spotlight to find it) and type this:

sudo codesign -fs - /Applications/Utilities/Boot\ Camp\ Assistant.app


It will probably ask for your password. Then it will say you need to download a developer tool from Apple. Agree to download it. (You don't need to install the whole X-Code, if it asks.) Once its been installed (its automatic), you need to go BACK to Terminal and run that command again. This time it will work.

After you've done that - open your hacked Boot Camp Assitant (it should run now) and have it make the Windows installation drive for you. Have the Windows ISO handy, and plug in a flash drive thats about 4-8 GB or more. The entire flash drive will be erased. This process can take a while. When its done, restart your mac and hold the option key. The USB drive that Boot Camp made for you will be there for you to install windows from. Its a yellow disk icon labeled EFI Boot.