If you are attempting to download SEP 12.1 off of FileConnect using an M serial number for SPS EE 3.0 or SEP 11.0, that won't do it because 12.1 is a major upgrade after SEP 11. You will either need the Version Upgrade notification or you can upgrade your license through the Licensing Portal. I know it is a bit complicated but once you have your Version Upgrade notification, you'll just want to keep that in a safe place.
Note, you aren't doing anything wrong but it can be a chore to get the right information.
If you still need assistance, please contact Customer Care at: http://www.symantec.com/business/support/assistance_care.jsp
Please let us know how it goes!