Your best bet is to politely contact your employer to find the status of the promised payment. Under the law you are not entitled to a severance. In order for a severance agreement to be binding, it must be supported by legal "consideration", which generally means a bargained for exchange. Most enforceable severance agreements make receipt of the payment dependent on your giving up any rights you have to sue the employer.
This is meant as general legal information and is not to be construed as legal advice in an attorney client relationship.