If you purchased a DVR without hard drive, you can install a hard drive by yourself. It's easy and fast.
Use your screw driver to open the DVR case.
Connect 2 Cables with your Hard Drive. (One is for power, another one is for signal)
Use Screws to mount the Hard Drive in the DVR's Hard Drive slot.
Close the DVR case.
Turn on DVR, follow the screen to do format hard drive. (only 1 or 2 clicks)