[O] latex export settings in init files

[Chris Beard]

Hello,
I've tried to modify some default latex export settings based on info from here
http://www.mail-archive.com/emacs-orgmode@gnu.org/msg07645.html

I basically add to the org-mode-hook to do:
(setq org-export-latex-classes (cons '("myarticle"
                                         "% BEGIN My Article Defaults
  \\documentclass[10pt,letterpaper]{article}

  
\\usepackage[letterpaper,includeheadfoot,top=0.5in,bottom=0.5in,left=0.75in,right=0.75in]{geometry};;
more settings after this...
    )
                                       org-export-latex-classes))

This works if I evaluate it after I've been exporting to latex, but I
get an error whenever I start up emacs:

setq: Symbol's value as variable is void: org-export-latex-classes

I'm guessing there's some org-mode latex-export thing that I need to
load first, but I'm not very familiar with how to do this. Any advice?

Thank you
Chris