Hmmm... haven't checked out ATX's to see if they unscrew the same way.
Make sure you're gripping the right place - assuming ATXs are like TXs, only the faceplate unscrews - the sides don't. A flat palm pushed against the faceplate and then turning it anticlockwise usually does it.
If not, there's a special Apeks tool that fits into the holes in the faceplate - they won't sell it to you, but you can jury-rig one up without too much hassle...
A few mug shots of a TX100