Installing an older version

I just switched to a Mac, so I have to learn details of the  
infrastructure.
I immediately installed R 2.6.1.
I also installed SPSS 16.0 on the machine.
As probably most of you know SPSS now interoperates with R, but  
currently it only seems to work with R 2.5.1.
When I tried to install R 2.5.1, the installer would not allow me  
because a newer version is already installed?
How can I install R 2.5.1 and keep my 2.6.1 installation?

Erich