[R] install problem 2.6 mac os x 10.5

[Emilio Gagliardi]

Hi everyone,

I am trying to install R-2.6 over my 2.5.1 installation on Mac OS X  
10.5.

R version 2.5.1 (2007-06-27)

I double-click the installer package,
1) Click Continue
2) Click Continue
3) Click Continue + Click Agree

On the "Installation Type" dialog screen, all four options are grayed  
out, with skip beside them.  The Install button is also grayed out.   
If I click the "Standard Install" button, it allows me to change the  
directory to install onto, but then brings me back to the same screen.

So I can't install.  If I click any one the four options [R Framework,  
R GUI, etc] the message says "There is a more recent version of this  
software installed on your system.  There is no need to install this  
older version."

What am I missing here?

Thanks for your help.