The main ClockGen window shows available functions on your system, therefore the layout may depend on your system specifications. If ClockGen is able to find a clock generator on your system, the buttons "PLL Control" and "PLL Setup" are present.
You first have to select the clock generator (PLL), so click on "PLL Setup".
In some cases, ClockGen will be able to detect the PLL that is used by your system, especially if your mainboard is equipped with an nVidia chipset. In this case, you won't be able to change the PLL model, and you can directly go to the "PLL Control" step.