Who Conducts the Arbitration Proceedings?
If you had an account(s) with any brokerage firm, you will be required to arbitrate through the Financial Industry Regulatory Authority (FINRA). In every new account form for any broker dealer, there will always be an arbitration clause. So, if you signed a new account form, physically or electronically, you agreed to arbitration. FINRA is the Regulatory body that oversees all registered brokerage firms.
If you were dealing with a Registered Investment Advisor (RIA), most of which are not covered by FINRA, most likely you will have to arbitrate through the American Arbitration Association (AAA).