You should start by asking your employer for a corrected W-2. Everything of value that you receive and compensation for your services is considered wages, even if it is not cash, and the value is subject to reporting on your W-2, and mandatory withholding of Social Security and Medicare tax. By not including the income on your W-2, your employer is intentionally or through ignorance, failing to pay their obligations for Social Security, Medicare, unemployment, and other taxes.
if the employer will not issue a corrected W-2, the correct course of action, for you would be to report the income using form 8919.