The Stasis Field comes with the DLC, but Slowdown has to be obtained and bought from level 3-1. After sliding down the beginning, stop the chariot on the disappearing death rail, jump into the alcove to the left, and be prepared to catch the chariot when it falls.
Once that's done, you'll need a second controller to activate both gadgets. Equip both, then trigger Stasis Field, then Slowdown. Your profile doesn't have to be the one using Slowdown, so it should unlock for both at once.