I don't think there's anything around on the mac that will completely duplicate IdeaMason (though I'm seeing this program for the first time).
Possible suggestions for alternatives include:
Scrivener
Dex (a new project-based materials management by the developer of Journler--it's still in development.)
Tinderbox (in many ways more powerful than IdeaMason. It's been called Photoshop for words. But that's its real strength: text. Not adept at manipulating or storing web or pdf or image resources.)

Of course, you could always try IdeaMason running in Parallels.