You can add on Part B right now. You may have a penalty to pay each time since you didn't add it when you were 65. You need to do this right away. I believe the enrollment period ends the first week in December. Medicare and SS don't really have anything to do with each other other than you can have the payment taken directly from your SS check. You'll have to pay the Part B premium by check or credit card until May. Then they can deduct it from your SS check if you wish.
You wouldn't be able to start Part B in May. You would have to wait until the fall enrollment period next year if you don't enroll now. The initial enrollment period (when you turn 65) is the only time that enrollment is different. Changes in Medicare have to be made during the fall enrollment period.
I assume that you didn't elect Part B right away because you still had employer based insurance. If this is the case, you can elect Part B when you retire. If you want assistance in making choices, call the local Area Agency on Aging. They have good staff to help you.
You wouldn't be able to start Part B in May. You would have to wait until the fall enrollment period next year if you don't enroll now. The initial enrollment period (when you turn 65) is the only time that enrollment is different. Changes in Medicare have to be made during the fall enrollment period.