I believe the custom label definitions are now stored in an XML file called pg_custom.xml .
Here, (Win8.1) this file is in c:\Users\myusername\AppData\Roaming\Microsoft\Office where "myusername" is a username constructed by Windows.
I do not know what else might be stored in this file. If it's just the label definitions, you should be able to copy that file. If it's other stuff as well, you may need to be careful just to copy the chunks of the file that pertain to labels.
I don't see this information in the registry either, but it's possible that it is now stored in a binary format key somewhere.
Peter Jamieson