@dsf
Basically you must have email template created I think. If you check installation files.
INSERT INTO /*TABLE_PREFIX*/t_pages (pk_i_id, s_internal_name, b_indelible, dt_pub_date) VALUES (22, 'email_warn_expiration', 1, NOW() );
INSERT INTO /*TABLE_PREFIX*/t_pages_description (fk_i_pages_id, fk_c_locale_code, s_title, s_text) VALUES (22, 'en_US', '{WEB_TITLE} - Your ad is about to expire', '<p>Hi {USER_NAME},</p><p>Your listing <a href="{ITEM_URL}">{ITEM_TITLE}</a> is about to expire at {WEB_LINK}.');
/*TABLE_PREFIX*/ Should be changed to your prefix (in config.php) and run on database.