Upon entering Impression (or Glow) you will automatically see presets. To access the custom sliders, press the P key. Once within the parameters, you can move the sliders around to customize the effect or create your own by pressing the reset icon.
After you've altered the effect or created your own, to save a preset, press the '+' icon in the top right menu.
ADD A NEW PRESET:
A screen will then pop up to name your preset and save it to an according collection.
The custom preset you created will also be saved to 'My Presets'. You can delete the preset simply by clicking on the preset and then clicking on the trashcan icon.