from an Adobe Tech Sheet:
Photoshop CS5 can be run as a 64-bit application on Mac OS, 10.5.7 and higher. Because of the changes that were made to the application, some plug-ins are not available when you run Photoshop CS5 either as a 32-bit or a 64-bit application. To work with these plug-ins, run Photoshop CS5 as a 32-bit application, then when you are done using these plug-ins, return to working in Photoshop CS5 as a 64-bit application.
If you copy old plug-ins from previous versions of Photoshop, or third-party plug-ins that are not compatible with Photoshop CS5, into Photoshop CS5's Plug-in folder, you may see different problems, such as errors or crashes.
If you have set the Additional Plug-Ins Folder in the Plug-Ins preferences to a folder, deselect this option, or make sure all the plug-ins in this folder are compatible with Photoshop CS5.
The easiest way to determine if third-party, old plug-ins, or plug-ins from previous versions of Photoshop are causing problems is to move them all into a new folder called "3rd Party plug-ins."
Plug-ins not supported by Photoshop CS5 on Mac OS, running in 64-bit mode:
To run Photoshop CS5 on Mac OS as a 32-bit application, select the Adobe Photoshop CS5 application file, installed by default in the Applications/Adobe Photoshop CS5 folder. Choose File > Get Info, and select Open in 32 Bit Mode. Deselect this option to run Photoshop CS5 in 64-bit mode.
To determine if a plug-in causes an error or crash, remove all old or third-party plug-ins from the Adobe Photoshop CS5 > Plug-Ins folder, and deselect Additional Plug-ins Folder in the Photoshop > Preferences > Plug-Ins preference.
It shows up, but only AFTER you have a selection active....You have to make your selection first. If it still doesn't show up, you can go to SELECT---REFINE MASK .... and you will get the dialog box.
I'm still having problems, but primarily have been trying with one or two photos. I end up with transparencies and the hair I "painted in" is gray...not brunette...as it should be. I've done Lynda's tutorial and it works perfectly, so I don't really know what's going on. The tech tried with the same photogaph and he said (just tonight...phoned back)...that his turned out worse than mine....sooooo...I'm really in a quandry here.
Let me know if it works for you. I'd be anxious to find out the results you get with the mask....refinement tool.
Good luck, sweetie