I am in the same boat as you. I updated to El Capitan and my Mbox USB interface is no longer recognized.
Jun 15, 2017 Mbox 2 Drivers; Manually uninstalling Mbox 2 drivers on Mac: Some Mac computers that have Core2Duo processors will experience problems with the Mbox 2 from Digidesign with the latest driver. To restore functionality, you will need to manually uninstall the Mbox 2 drivers and install version 10.3.3. Follow the instructions below to manually remove the drivers. Thanks for reaching out to Apple Support Communities. I understand you’re having issues using your Digidesign Mbox 2 Mini with your MacBook Pro. I’ll be happy to help. If the light to your device isn’t coming on when you connect them, go through the steps in this link: If a USB device doesn’t work with Mac.
If you scan the forums you will find that other MIDI interfaces appear to have stopped working as well, it is not just the Avid MBOX 2. This points to the issue being on Apple's side, but I can't say for sure..
I am hoping for a fix from Apple in the next point release as I am mid-project as well...
Driver For Mbox 2 Mini Mac Driver
In the meantime I would recommend submitting a bug report to Apple. Hopefully the more of these they get the more likely we will see a fix..