If you have trouble because of RGtk2 (e.g., it’s saying you don’t have gtk2.8.0 or higher while you have one already). If so, follow this procedure.

(Note: Don’t use macports! It won’t work with RGtk2 properly.)

Instead, install R from r-project.org. This is pretty much important. Don’t install R from macports.
Install R from http://www.r-project.org/.
Install gtk2 from http://r.research.att.com/#other

Now, install RGtk2 should work fine.

> install.packages("RGtk2")
> install.packages("rattle")

Make sure that your LANG is English. Otherwise, fonts in the rattle will look broken. Set this in your ~/.profile:

export LANG="en_US.UTF-8"

In R,

> library(rattle)
> rattle()

Now it’s all set!