This has had me baffled for a long time. When I'm travelling, I use my iPhone 5 as my Internet connection for my laptop. I have no problem tethering via Bluetooth, and almost no problem* tethering via wifi. But I've never been able to tether via USB. When I go into network prefs on my Mac, the iPhone USB never appears as an option. I recently saw an article online that said for some peculiar reason, you must have iTunes running before you attempt to tether via USB. I tried this as well, but still no luck. The iPhone USB option never appears in the network prefs. I'm running iOS 7.1.2 on the phone and OSX 10.6.8 on my Mac. Tethering has been available since 10.5.x.
* - When tethering via wifi, the iPhone will often not appear in the network list under the Airport menu. I'll have to go into the iPhone's Settings, and toggle Personal Hotspot off and on, before it appears in the Mac's network list. After that, it works fine.
