You may not need to download anything depending on what controller you have.
For mission planning, there is nothing to download. Go to Litchi’s web site using your favorite browser and plan a mission using the Mission Hub.
For a Mini 5 Pro, the missions you plan would be flown using DJI Fly. You will need to transfer the mission from Litchi’s Mission Hub to the device running DJI Fly.
This thread should have all the information you need for using the Mission Hub and transferring your waypoint mission: