Plug HTC Evo Design 4G (Boost)

Hey, I have a HTC Evo Design 4G from Boost. I'm trying to unlock the bootloader so I can root it. I have HTC Sync, which installed the lastest drivers apparently on Windows 8.1 Pro, and it connects when it's on, but when I follow the method to get into the fastboot option on the phone Windows says the device is not recognized and fastboot.exe on the PC cannot find the phone now. I need help, please.