[R-SIG-Mac] Problem Installing 2.6.0 on Mac

[Jason Horn]

I'm not able to install R 2.6.0 on my Mac running 10.5.  When running  
the installer, and after agreeing to the license, in the "Installation  
Type" stage, the Packages are all greyed out (R framework and R GUI  
for MAC OS X), and therefore the "Install" button in the lower right  
is greyed out.  It won't let me select anything to install.  Anyone  
know what the problem here is?

- Jason