Axe-Fx II USB driver for Windows and Mac

USB drivers for Mac and Windows are required and available for download here. If the USB driver is not installed, utilities such as Axe-Edit, Fractal-Bot and Cab-Lab won't recognize the Axe-Fx II.
When using the USB interface, there's no need for an additional audio interface.
An external audio interface will be required when connecting the analog Axe-Fx II outputs to a computer (not using USB).
A MIDI interface is not required to connect the Axe-Fx II to a computer. The Axe-Fx II itself operates as an USB-to-MIDI adapter. MIDI interfaces can still be used, and editing through a MIDI interface also is still possible.
Cliff: "The driver that we supply for OS-X computers is NOT an audio driver. It is a firmware installer. The Axe-Fx II uses a "soft" USB controller. It gets its code from the host computer. When you turn the Axe-Fx II on it requests firmware from the host. This is superior to a hard-coded controller in that updates merely require a new host image rather than reflashing the controller."

Using the Axe-Fx II as an interface to update the MFC-101 firmware

The Axe-Fx II can be used as an USB-to-MIDI interface to update the firmware on the MFC-101, using Fractal-Bot or a MIDI librarian. Walkthrough.
Cliff: "The USB Adapter feature was added for simple USB-to-MIDI functionality, specifically firmware updates for the MFC-101. It is a low priority task since audio processing is the Axe-Fx's primary usage. Latency is not guaranteed. It is not intended as anything more than a no-frills, simple MIDI adapter for non-critical tasks." source

Creating an USB connection increases CPU utilization. This is by design. If a preset uses much CPU, you may run into CPU overload problems when connecting the Axe-Fx II to a computer through USB. source and source

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High-pitched noise may be caused by a ground loop.
- Cliff: "The USB spec calls for grounding the cable at both ends (mistake). This can create an USB ground loop. Make sure everything is plugged into the same outlet strip." source
When experiencing problems such as dropped connections or an Axe-Fx II which freezes after connecting USB, always replace the USB cable first. Example
Cliff: "An occasional flicker when hooked up to USB is not unusual. This is due to the screen redraw being interrupted by the USB thread." source
Handy tip for resolving audio dropouts in your Windows DAW

The Axe-Fx II works as an audio and MIDI interface just as on Windows or OSX, although Linux is not supported officially.
As explained by Cliff (see bullet 5 of the section "Axe-Fx II USB driver for Windows and Mac") the Axe-Fx II uses a "soft" USB controller. This controller needs to be loaded with its own firmware. The way to do this on Linux is using a tool called "fxload" executed by an appropriate UDEV rule. There is a BASH script available which generates and installs such a UDEV rule. A package containing the script and the mentioned USB controller firmware file is available for download here. The additionally required tool "fxload" is typically not preinstalled by current Linux distributions but needs to be installed by the user.
After the USB controller got loaded with its firmware the Axe-Fx II will be recognized as a USB soundcard. It will be reported as a USB device with Vendor ID 2466 and Product ID 8003. For a named output adequate entries for Fractal Audio Systems and the Axe-Fx II need to be added to a file called "usb.ids". source
Since the Axe-Fx II is an Audio Class 2.0 compliant device no audio driver is required. (For systems with a kernel older than 3.10 read further instructions here).
The ALSA usb-audio driver may cause overflows or underflows since there is no proper sync applied. No fix has been merged on linux upstream yet. See here for a patch and instructions on how to build the snd_usb_audio kernel module.
Threads about using Axe-Fx II with Linux: