If you get a message saying /System/Library/Extensions/AppleUSBEthernetHost.kext was not installed properly and needs to be reinstalled here is what you need to do:
Go to the location indicated above and trash the file AppleUSBEthernetHost.kext.
Go to http://www.apple.com/itunes/download/ to redownload iTunes 11.4 as the first install damages this file for some users.
Install iTunes again and if you check where you deleted the above file, there is a new one there and everything is fixed.