Cant boot windows 7 on my PC I just built

I recently built a computer, I can load the BIOS but when I plug a USB device into it and boot from that all I get is a black screen with a blinking underscore.

 

(All I have is a mac so no .exe app for fixing)

did you buy the usb from microsoft or did you put the OS onto a usb of your own? you might not have formatted it correctly