It looks simple, but I have received many queries. Here's a short tutorial on how to do it. A similar process is also for Axe-Fx II and III.
1. Open AX8-Edit and choose Tools - Axe-Manage Cabs
2. Go to the IRs folder and select all
3. Drag & drop all IRs to the appropriate position in Axe-Manage Cabs & press SAVE. When the import is complete, press CLOSE. If you purchased presets that contain user cabinets, these cabinets must be imported first and in the right position to make everything work smoothly. Otherwise, there will be no sound. I always attach a short txt file to the IRs folder where the exact import position is mentioned.
You have successfully imported user cabinets! Enjoy. :)