How to diagnose why plugins don't show in Photoshop? I have some Photoshop plugins in a folder PSPlugins in my home folder.  When Photoshop runs, it doesn't see them.  I did put the correct path into the preferences.  Is there some way to determine why Photoshop doesn't see these plugins?  Does it really not see any plugins, or does it see them but doesn't like how they taste?  

Does PS write a log file stating where it looks, what it finds, and any error messages?
One thing to try is to find the Photoshop application, and Get Info on it. You can do this by selecting it in your Applications solder and pressing Command I or right clicking and choose Get Info. If you see a check box marked Open in 32 bit mode try selecting it, and then launch photoshop again.

Plugins for all programs generally need to be 32 bit if the host app is running in this mode, or 64 bit if the app is. 

This is from experience with Logic plugins, which would refuse to acknowledge some older 32bit plugins when running in 64 bit mode.

Having typed this, I've since found this relevant page: http://kb2.adobe.com/cps/828/cpsid_82824.html#main_32%2064%20bit%20launch