You will need to call getAllRooms() method of WarpClient. The result will be provided in onGetAllRoomsDone callback.
Regarding when to call, it depends on your game play. You can call this method any time from any where. WarpClient is a SingleTon class. Hence you can call it from any script. When the result comes in onGetAllRoomsDone (which in our sample is defined in listener.cs) you can use that information to display it on the screen.