Maybe a Numbers version earlier than 2.3???
In any case if export is really gone from the menu a likely workaround is select the cells in the table you want to export, command-c, command-v into TextEdit, save as tsv (tab-separated-value) file.
Thanks for the help. For whatever reason Software Update gave me nothing, but I was able to do the manual download/install for iWork 9.1 (the last version supporting 10.6.8) and I now have a CSV export option.