The 3rd option in the buying guide table includes Master Collection CS5 as an upgrade path.
@tgms: if Master Collection CS5 is already installed on the computer where you're trying to install MC CS6, the CS6 installer should find CS5 automatically and not require you to enter your CS5 serial number.
Call Adobe Customer Service in your country. User to user forums cannot help you with serial number issues. Only Adobe themselves can do that.
Problem is, I don't have the option to select CS5 in the list when I input the existing key. Only CS5.5
Contact support by phone or web chat.