Trying my first beamer document
I was getting this error message:

  No definition for class `beamer' in `org-export-latex-classes'

I tried Carston's example.. same error.

Looking at the org-export-latex-classes, I found indeed that there was 
not specification for 'beamer.' However, if removed customization for 
this setting, the class appeared.

So I searched and searched through all of my init.el but found nothing 
that might have overwritten the export-latex-classes.

Restarted emacs and tried Carston's example.. IT WORKED just fine.
Sure enough, beamer was defined.

Conclusion: it must be something in my own document that was mucking up 
the class list.

And it was here:

   #+LaTeX_CLASSS_OPTIONS: [presentation]

see the error? CLASSS (too many S's)

It only took 2 hours to figure this one out.
-ben
-- 
Capt. Ben Smith
R/V Coastal Surveyor - R/V Cocheco - S/V Mother of Perl
Center for Coastal and Ocean Mapping Joint Hydrographic Center
University of New Hampshire, Durham, NH, U.S.A.