I have PS CS6 with the latest updates, using a Mac Pro computer with OS 10.7 also with the latest updates. When in ACR, the adjustment brush does not appear on the screen unless I click the bracket keys which increase or decrease the size of the brush. If I paint a segment and the let go of the brush key, the brush disappears again. The adjustment effects work properly once I have brought up the brush by pressing the bracket key. I have to keep re pressing the bracket key if I am painting adjustments on multiple parts of the photo. I have restarted Photoshop and the computer numerous times, thrown away and reset the preferences for both Photoshop, Camera Raw, and Bridge, as well as repairing disk permissions on my Mac. Any suggestions?
If it was a PC I'd suggest seeking an updated display driver, because a missing or partial cursor is a pretty well-known sign of a display driver glitch. As it is a Mac you pretty much have to wait for a display driver to come in with an OS update. You might want to let Apple know that it's doing what you're seeing with your particular hardware.