If your client doesn't print out its join key, Devon suggested: "You can try to push the button 1 on the client starter kit board, it should display the join key on the serial 1 console."
This worked for me!
Where can i find a manual of the Thread Stack CLI? I searched in the huge SI Labs document library but i couldn't find it.
Check out protocol/thread_x.x.x/app/thread/doc/README