I had the same issue, see this workaround, here:
Configuration Manager Snapin dropdownlists are broken: empty and gray.
Choose the Workarounds (1) and it reads:
Do not use the version of SQLServerManager11.msc in the C:\Windows\System32 directory on Windows 2012 64 bit (possibly extended this to any 64-bit OS?) the drop-down boxes are gray and inaccessible for the options.
Instead use the version in C:\Windows\SysWOW64 as it appears to avoid this issue.