Unfortunately, there is probably not much that you can do. That is unless not doing so violated company policy or there was a union agreement or employment contract that required you be shown such a complaint. The fact is that in an at will employment relationship, an employer can set the terms and conditions of employment as it sees fit; this includes those terms and conditions surrounding termination. And the fact is that an employee can be fired for any reason or no reason at all.So unless your treatment was the result of some form of actionable discrimination or retaliation, you had no right to see the complaint that was lodged against you. While it would have been preferable, it was not legally mandated.