Your post is missing lots of information and its not exactly clear what you need. Can you repost with mor details.
READ THIS BEFORE CALLING OR EMAILING ME: I am licensed to practice before the state and federal courts in Virginia. We have not established an attorney-client relationship unless we have a signed representation agreement and you have paid me. I am providing educational instruction only--not legal advice. You should speak with an attorney to whom you have provided all the facts, before you take steps that may impact your legal rights. I am not obligated to answer subsequent emails or phone calls unless you have hired me. I wish you the best of luck with your situation.
Your starting point is your Final Decree. Your second step is reviewing any Property Settlement Agreement you may have entered. Your third step is contacting the plan administrator. Your fourth step is taking all of the relevant paperwork to a local attorney.
My answer is general in nature, please do not rely on it as your sole resource for this very important issue.
You need a lawyer to do what needs to be done, and you would be wise to hire one quickly. Don't even think of doing it yourself. You may have even been entitled to start collecting your share when your ex turned 55, depending on his plan.