How to add Spanish Dictionary to an English Ubuntu installation

You have to install the package openoffice.org, and the myspell-es with the command

sudo apt-get install openoffice.org myspell-es

in addition to common installation of open-office.org-writer and open-office.org-calc, etc.
It's a metapackage that installs all components of openoffice.org, besides standard ones, and that's what make spanish dictionary available.
If that doen't work, you might want to try the wizzard inside Open Office's menu:

File/Wizzards/Install new dictionary