Which version of mac os x are you using?
I don't think it should make any difference, but are you running cs5 in 64 or 32 bit mode?
Does using one mode or the other make any difference?
If you go to /Applications/Adobe Photoshop CS5/Presets/Scripts, do you see a Photomerge.jsx and a Stack Scripts Only folder?
At the bottom of the File>Automate menu does it say Show All Menu Items?
Could you post a screenshot of the File>Automate menu in photoshop cs5?
I am using OS X 10.9.4 (13E28)
no difference in mode, the photomerge is not there
there is a stack scripts folder
there is not a Photomerge.jsx folder
the bottom of the File>automate folder does not show all menu items
here is my screenshot
Your help is very much appreciated
Sounds like a reinstall of photoshop cs5 is needed if you don't see the Photomerge.jsx in the
/Applications/Adobe Photoshop CS5/Presets/Scripts folder