That's because that theme is booby-trapped - the developers of the theme have done that so that you cannot change that part of it. We don't support themes like that here - they are not in keeping with WP GPL licensing requirements/policy and philosophy. You should find another theme ASAP.
This explains a bit more - http://www.chipbennett.net/2010/12/10/only-download-wordpress-themes-from-trusted-sources/
