This is just great. It has come to my recent discovery that a Macintosh equivalent to Windows's RPG Maker has been released on the Japanese side of the net. So all you other Mac users should be shouting "HURRAY" right about now.
It's called miRPG Maker, and it's a LOT like RPG Maker XP for Windows, albeit differences. You can snag it here: http://www.vector.co.jp/download/file/mac/game/fh456101.html
Download usually starts automatically, if not, click the hyperlink DIRECTLY under "( Filesize: 7,501,288Bytes )".
Surprisingly, it's all in English, so don't worry. Also, the creator builds actual full APP files, so they can be played as a whole and circulated easily. Have fun with this.