When Windows installer asks where to install Windows 8 you chose the BOOTCAMP Partition.
Installer says it can't install to that partition because it needs to be formated to a format Windows 8 can use.
There should be an option button near the bottom which brings up a new window with the option to format the partion. Select the BOOTCAMP partion and select FORMAT and use NTFS.
I am writing this from my memory so the Option button may be called something else but you need to get to the formating window.
After formatting you should get a NEXT button and Windows 8 will begin the installation.
Let us know how it goes.