POSSIBLY HELPFUL INSTRUCTIONS FOR DOWNLOADING R COMMANDER FOR MAC USERS
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I will be adding to this page if I find any more useful tips.

This assumes you have ALREADY downloaded R itself and have typed:
install.packages("Rcmdr", dependencies=TRUE) 
and chosen a mirror site.

Tip 1:

If you see an error message like:

"Error: package 'tcltk' could not be loaded" 

or

Warning message:
In getDependencies(pkgs, dependencies, available, lib) :
  package ‘tcltk’ is not available


First try typing
install.packages("tcltk")
and see if that enables you to call up Rcmdr by typing:
library(Rcmdr)

You may have to first go to the web page:
http://cran.r-project.org/bin/macosx/tools/
and click on the link labeled
tcltk-8.5.5-x11.dmg
and THEN try
install.packages("tcltk")

Tip 2:

After clicking on the <tcltk-8.5.5-x11.dmg> link and then typing
install.packages("tcltk")

then click on the X11 icon (it should look like a box with a black X on it
near the top of the R window).  A separate xterm window will pop up.  
Don't close this window, but you can ignore it.

Then type in the main R console
install.packages("Rcmdr", dependencies=TRUE) 
and choose a mirror site.  The Rcmdr package should download.
Then type 
library(Rcmdr)
and the R Commander window should pop up.

NOTE:  Whenever you want to use R Commander in future sessions, you should open R
and click the X11 icon and then type
library(Rcmdr)

Hopefully that will work!