You need to find a way to make an ISO.
Maybe try AnyToISO or the method below.
1) Go to Finder > Utilities > Disk Utility (or Spotlight > Disk Utility)
2) File > New > Disk Image from Folder
3) Select The Windows 8.1 Pro Folder and click "Image".
4) In Image Format choose "DVD/CD Master" and in Encryption choose "none". Then click "Save". (Save it on desktop).
5) Next go to Terminal, Finder > Utilities > Terminal (Spotlight > Terminal).
6) Use the code below
CODE
cd ~/Desktop
hdiutil makehybrid -iso -joliet -o sample.iso sample.cdr
Change sample.iso and sample.cdr to your target file name. Then enter.
7) Now, try opening Boot Camp Assistant with your USB Flash Drive plugged in and try installing. Hope that works.
I'm downloading the ISO at the moment. Wish me luck tonight on installing it T.T