To activate the challenges, you first have to pick the difficulty with one of the following commands:
/function easy
/function medium
/function hard
/function impossible
Then, select 1 or more of the challenges that you want to enable.
/function lavarising
/function waterrising
/function manhunt
/function hardmobs
/function fallingtnt
/function fallinganvils
The other commands you can do are
/function reset - resets the challenges
/function disablemessages - turns off messages from challenges