Are you using the LTC6948 eval board, FracNWizard, and DC2026 (USB to SPI board)? Or are you trying to write to the part using your own software and hardware? Can you send a plot of your SCLK, SDI and !CS signals, so I can look at the timing?
To write to the device please follow this timing diagram. Notice after Address bit A0, there is bit that indicates whether the SPI is in Read or Write mode.