For some weird reason, activating the spellcheck on OpenOffice 2.4 seems to be more challenging than it should, and since I just got it working in Ubuntu Hardy Heron, I thougth I should post the solution here:

1. Go to Tools > Options…
   

   OpenOffice tools menu with options entry highlighted

2. Select Language Settings > Writing Aids and click on the Edit… button
   

   Writing aids dialogue with edit button highlighted

3. Check the boxes for the Spelling and Thresaurus modules. This will enable the spellcheck for the language in the Language field, so you should repeat this process with each of the languages you want to use.
   

   Language module editing dialogue with spelling and thesaurus entries highlighted

4. You should now see the spellcheck symbol beside your chosen language in the language settings dialogue at Tools > Options… > Language Settings > Language. Enjoy!
   

   Language settings dialogue with spellcheck symbol highlighted