In order to install windows 7 you need to use the USB 2.0 (the 2 USBs from the back) at this moment the NUC does not have the 3.0 USB driver.
But before to install the OS you need to make a few changes on the BIOS configuration in order to install the OS windows 7.
>Press F2 when prompted during boot to enter BIOS Setup.
>Press F9 to set all BIOS options to default values.
>On the Devices and Peripherals > SATA menu, Chipset SATA Mode is set to AHCI.
>On the Boot > Boot Configuration menu, in the UEFI Boot pane, OS Selection is set to Windows 7.
>Press F10 to save and exit BIOS Setup.
Proceed with the installation of the operating system.