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?

You can try the following. Whether it works depends on the exact OS X  
version you're using:

1) rename your current R.app (in Applications) to anything else (e.g.  
R-26.app)
2) remove R installation receipts:
rm -rf /Library/Receipts/R-*
3) install old R - make sure it says "Install" and not "Upgrade"

Now you can use Rswitch from http://r.research.att.com/ to switch  
between the installed R versions (this may be relevant to SPSS etc.).

If you use GUI, you don't need to switch the version, the GUI loads  
the version it was built for, so you can use both icons - the R-26 and  
R.

Cheers,
Simon