I need to reference SPSS in an article I am writing, how should I do that?

The following information is taken from official SPSS FAQ:

For most of our products, you can get the complete product name, version or release number and date of creation from the "About . . ." dialog box that appears under the "Help" menu. For example, if you have SPSS for Windows, you might see this:

SPSS for Windows
Release 10.0.0 (September 1999)

You can then write a bibliographic citation as follows:

SPSS for Windows, Rel. 10.0.0. 1999. Chicago: SPSS Inc.

But hold on! The latest official information from IBM on citing SPSS is available here.