If you are running a cron file in crontab then. including WP files will not help as it doesnot run as a wordpress file. it is like a simple php script. You will not get access to access wordpress functions. you will have to run php script and connect using php connection to the database.
Some servers which host wordpress and support will execute. for them you will have to provide path to the files of wp-config and includes form the root level.