install error on mac

Hi,

Is this a common problem when installing matplotlib on Mac OS?
The matplotlib (OS X/.dmg) installer provides this error msg:

You cannot install matplotlib 0.99.1.1-r7813 on this volume. matplotlib
requires System Python 2.6 to install.

···

##

Yet it appears I have the correct Python available on OS X 10.5.8:

$ whereis python
/usr/bin/python
$ python
Python 2.6.3 (r263:75183, Oct 4 2009, 16:48:48)
[GCC 4.0.1 (Apple Inc. build 5484)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

##

Thank you for any advice.
Seeking a popular method to install all the bits...

Cheers,
DK