to active the autosave you need first to find this line in the caster config:
# autoSave enables or disables automatic replay saving after a match is
# complete. The replay is saved IMMEDIATELY after the match is done, so you
# can't turn this on during the win screen, sorry.
#
# You may enable or disable this while MBAC is running by pressing = or -.
autoSave = 0
That 0 change it for 1 and it will automatically save any fight you play or watch in a replay.