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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.