Employment Law Questions? Ask an Employment Lawyer.
Unfortunately, in terms of registering for Medicare Part B, there is not much you can do to rectify that issue. Medicare Part B only has an open enrollment period that begins on July 1.
What you would need to demonstrate thought to have a case against your employer is if there is an indication to believe that you wouldn't be affected by the opt-out of your retirement but that you actually were. Unfortunately, without some actual fraud taking place, there isn't anything you can do in terms of suing your previous lawyer.
Have you looked into the healthcare exchanges provided by the government at www.healthcare.gov/retirees. Despite there being an open enrollment period, there is a special enrollment period if you just lost your job-based coverage.
That special enrollment website can be found at: https://www.healthcare.gov/coverage-outside-open-enrollment/special-enrollment-period/
Even if you do not qualify for the special enrollment period, the open enrollment period begins November 1, 2016.