Hi Jing,
You can set the clock rate in DIAdem DAC by using the block "system clock" (first toolbar, looks like a yellow metronome). If you double click it you have several settings (it depends on the hardware which settings you can use). For NI hardware you can set ->"DAC kernel" -> "Hardware clock" and "Clock rate". Than you must connect the (yellow) block output with the corresponding input (yellow icon) of all other blocks (which require this connection) in your DAC scheme.
I hope this can help you
Greetings
Walter Rick