[R] cannot install R-2.6.1 on Mac OS X 10.4.9 PowerPC G5

[shelley]

Hello:

I tried to install the latest R2.6.1 on my Mac OS X machines. I have  
no problems installing it on my Mac OS X Tigr machine. But when I  
tried to install it on other Mac OS X 10.4.9 PowerPCs, I could not  
have the GNU Fortran and/or Tcl/Tk libraries installed. I always got  
the error message : There were errors installing the software, please  
try installing again.

Does somebody have any idea what was going wrong? I had X11 installed  
on the machines, do I need to install other softwares before  
installing R?

Thanks!
-Shelley