Can not install Windows 8.1

Easy. You can install it like you would on any other PC.

WARNING: BACK UP ALL DATA BEFORE TRYING!

First I suggest you test it. Reboot, insert disk, holt option key, and select Windows disk to boot into. If it works and you can browse the setup, then it's good and it should work.

Now you need to partition the HD to install windows on.

I'm not sure if you have to make a partition with NTFS or any format. In the Windows POSReady2009 installer, the format of the partition doesn't matter, as long as it's a partition; the installer formats it itself. I don't know if the 8.1 installer is the same but it probably is. You can try it out.

In OS X, go in Disk Utility, select your HD, and create a new partition by shrinking the main one (drag it up) and clicking + to create a new one. Make it the size you want, but the name and format doesn't matter, assuming the installer will reformat the partition anyway.

Now once the partition is made, boot into your installer disk. When you reach the screen where you choose Easy or Custom install (or something like that) CHOOSE CUSTOM INSTALL or you may erase all your data. You should see a partition screen. Select the second partition that you have made in disk utility. Look at the size of the partition to see if it is the one you created. Select it, format if necessary, whatever needs to be done (I don't know the 8.1 install much).

After you install it, you should have windows installed! But the screen may be low resolution and things may not work; this is because you need the drivers. Shut down windows and go back to OS X.

Now go here:
https://support.apple.com/kb/DL1720?viewlocale=en_US&locale=en_US
And download the bootcramp software. Follow the instructions it gives you there of putting it on a USB stick, etc.

Boot back into windows, open your USB stick, and install the Bootcramp software. Note: i'm not sure this software would work, but I think it should. If it doesn't then you can try an older version of Bootcamp (here). If none work then you might have to find individual drivers yourself... or else you can't use Windows fully... But I think the bootcramp sotware should work fine.

 

It will work on your machine, but Apple does not want it to work on your machine. Why? I don't know.

Cannot partition? In Disk Utility? File system doesn't match? That's weird... Might wanna try to repair the HD (select the HD and click Repair Disk; this will repair partitions, etc. Selecting the OS X volume will repair the partition itself, try that too).

But I think you should return it while you can. Apple, for some reason, doesn't want you installing 8.1 on your machine, only Windows 7, so they won't support it and just say that it doesn't work (when it actually does), so you won't get help for it. If you use 7 then it would be much easier.