If you mean "mis-mailed" to the wrong person, then you're out of luck. The person receiving anything addressed to them by mistake may keep it as a gift. Society believes this is a fair price to pay, so you won't do it again.
The insurance company will not be amused: you cannot collect insurance if there was no accident involved; this appears to be negligence which means it's your fault, not just some unforeseeable event or "act of god". They aren't "lost", just given away to the wrong person. The insurance company won't pay if you changed your mind and wanted to send these to someone else, and this is exactly the same thing to them.
The post office has no liability for delivering something to exactly the person you addressed it to, and they have no reason to know you made a mistake, and they are not going to help you reclaim the properly delivered package.
It depends on which insurance one needs. If it's just basic medicare, one can apply online at https://secure.ssa.gov/apps6z/iClaim/rib
You can apply for Medicare 3 months prior to your 65th birthday, the month of your birthday, and 3 months following your 65th birthday. Check out this page for information on Medicare: http://www.medicare.gov/Publications/Pubs/pdf/10050.pdf You should also know that the 2011 Medicare open enrollment dates are quite a bit different than in 2010. (This is the period when you can switch from Original Medicare to Medicare Advantage and vice versa ... or switch between Medicare Advantage plans.)