Boot Camp Assistant does not let me change my destination drive
Best way to do this, delete the FAT partition and re-expand your HFS+ partition to fill up the space.

Use bootcamp assistant to create a bootable USB key (you'll need an 8GB one). It will ask for the ISO to make the installer.

Once it is done boot from the USB key and follow the prompts. Bootcamp will partition your drive for you at the beginning of the setup process and set it up properly to dual boot.

My guess is that something in the way you made the partition (location, type, etc.) was incorrect. If you let BootCamp do it, it works great.