I was unsuccessful using the KEXT Utility and the older KEXT file that everyone else is using. I tried several times. It never worked to fix my USB Thether.
I'm on iOS7, iPhone 5S, originally Mac OSX 10.6.8, but then upgraded to Mavericks - problem happened on both OSes for me.
My Successful Steps:
1. Unplugged iPhone & deleted iPhone USB Network connection from Mac network connection
2. Went to iPhone - Settings - > General - Reset -> Reset Network Settings and rebooted phone
3. Downloaded full iTunes 11.1.5 installer and Pacificst free tool
4. Opened iTunes DMG installer with Pacifist tool (much safer feeling then KEXT Utility from Russia).
5. Drill down into the DMG with Pacifist through "Contents of Itunes" - > "Contents of manual" - > "Contents of MobileDevice.pkg" - > "System" - > "Library" - > "Extensions"
6. Open finder and press Command + Shift + G and enter path of "/System/Library/Extensions".
7. Click and Drag the AppleUsbEthernetHost.kext file from step 5 on top of the Finder window shown in the step above
8. It will prompt you to enter your admin password to overwrite the files. Do that.
9. Reboot your Mac
10. Connect your iPhone again
11. Activate mobile hotspot.
Viola for me after waiting since Oct 2013 to get it working right.