There isn't one. If you really want to make levels, you'll have to either:
1: FIND A PC.
2: Figure out how .j2l files are constructed and make your own level editor.
3: Figure out how .j2l files are constructed and make your own in a text program or a hex editor.
4: Tell someone else the level you want made, tile by tile, event by event.
5: Wait for someone else to make the exact level you had in mind - it should only take a few centuries.
Edit: 6: Remake JJ2 and JCS on your own for the mac, then make your level.