I recently bought a new 27" iMac running OSX 10.8.2 (Mountain Lion). I brought over all my files, including applications from my backup disk. I had no trouble running Photoshop CS5.5 and printing any pictures I had imported from my cameras. The Twain plugin that I use is the same one that I was using in my previous iMac running OS 10.6.8. I had a similar problem with it when I had updated from OSX 10.5 which was solved with updating the Twain plugin. There were some differences in that the scanner showed up in the import command, but could not be accessed. Here no scanners show up at all!
By using the EPSON Scan application, I can access and scan, producing a variety of file formats, but it does not integrate with the Photoshop program. Also, my OCR program is also dead in the water. I can also use the scanner from within the Print & Scan prefernce pane, but this only lets me get a jpeq result.
I followed all the directions regarding updating the scanner S/W and Twain plugin with no luck. I cannot access the sanner from within photoshop.
Moving the discussion to PS forum
You may have to run Photoshop CS5 in 32 bit mode if the Twain and Epson drivers are 32 bit.
Click on your Photoshop CS5 app icon to select it, context or right-click on it, select "Get info" or choose File > Get Info from the Finder menu.
In the dialog choose "open in 32 bit mode"
The 32 bit version should be able to use the Twain plug-in and scanner drivers.