[quote name='JJSP']Apparently Mac users have to use the developer method instead of the Flash-based creator. Any tips on using that route?[/quote]
The developer route is far superior IMO anyway. I usually just use the creator to try to line up text/gamer pictures/gamer tiles in the general vicinity they go in anyway. From there, I use the dev editor side to add things like custom rep, ect. and to line everything up completely, then save and preview to make sure it looks right. The creator tool lacks the ability for most of the customization anyway and it's rather difficult to line things up perfectly in it as well.
My advice would be to just tinker with it and learn the ropes and then as you get comfortable with that, start adding your own custom pieces. For instance, my current gamercard (in my sig) there is a custom overlay for gold/silver account status and custom completion bars, and custom status markers, but there is a bunch more stuff you can add. Just look in the help section and the basics for all that stuff is in there. If you get stuck, try their forums. They are pretty good for the most part.