It seems the problem is that that the 64bit version of Ruby 2.0.0 creates a registry entry in a location that the Devkit initilisation script (dk.rb) is not aware of ('Software\Wow6432Node\RubyInstaller\MRI').
Until the Devkit installer is updated, you can get the installation to work by opening %DevKitExtractionDir%\dk.rb in a text editor and changing the REG_KEYS array to include the 64-bit registry path, so it looks like this:
REG_KEYS = [
'Software\RubyInstaller\MRI',
'Software\RubyInstaller\Rubinius',
'Software\Wow6432Node\RubyInstaller\MRI'
]