Troubleshooting 

Set a class in the constant defaultCssClass first. It is added to every page, so you can see whether the extension renders the body tag at all. Reload the frontend and check the source code after every step.

1. Make sure the extension is installed and the database fields exist (Admin Tools > Maintenance > Analyze Database Structure).
2. Make sure the static template Pagewide CSS Classes (pageclasses) is included in your main TypoScript template (see Include the TypoScript). Check it with Included TypoScript in the TypoScript module (Web > Template in TYPO3 12, Site Management > TypoScript in TYPO3 13).
3. page.bodyTagCObject may be set by another extension or by your own TypoScript. Find out with Active TypoScript in the same module and assign the plugin at the very end of your setup, see Manual setup.
4. Your PAGE object might not be called page. Check this in the same module.