FreeGameDev

# When opening levels or maps from the in game editor,
# ST looks into this location for files:
~/.local/share/supertux2/addons/Forest_World_Adventure_Pack/levels/forest_world_adventure_pack/

# After saving edits they go into this location:
~/.local/share/supertux2/levels/forest_world_adventure_pack/

# Manually copy them back to where they started
# -- or you will lose changes when you restart the editor
cp ~/.local/share/supertux2/addons/Forest_World_Adventure_Pack/levels/forest_world_adventure_pack/*
~/.local/share/supertux2/levels/forest_world_adventure_pack/

... it took me awhile to figure this out. : -)
be sure to swap out the Forest_World add-on name with your own.