I'm not exactly sure what you mean. We will have an OS X Editor soon (currently at Alpha--which means it's a bit buggy--on Westlake's project list). This will mean that we can create maps and edit the rules (everything from the characteristics of each of the 16 civs, to how improvements and wonders works).
Of course, I'm assuming that our mac Editor will have all of the features (and some of Brad Oliver's creative enhancements) of the PC version. Hopefully, you will be able to mod to your heart's desire in a few weeks.
In terms of 'editing' other people's scenarios (the mythical Scenario Editor), I think that another sub-routine might be required. I would really support some kind of listing by a knowledgeable member of the Mac community that tells us which mods/scenarios that are available for the PC could also work for the Mac.
I hope a few others comment on your thread