Hi there,
I experienced what you guys have been getting, here's my work-around for it:
In ArcCatalog / Customize / Customize Mode /
1. click 'Add from File' button and pick the X-ray AddInFile
2. In the commands tab, find the recently added x-ray command tool and with the window to the right, 'drag out' the x-ray button to dock onto your tool bar.
3. go to Customize / Extensions / Click to make available X-ray Extension, which is now available.
4. Go back and remove the docked x-ray tool bar.
This should work, I've done it multiple times, good luck!!!
Jesse
GIS Technician / Town of Sidney, BC, Canada