Moving this discussion to the Photoshop General Discussion forum.
Depends. There are no 64 bit TWAIN plugins. You can get one in 32 bit mode.
Current advice is to use the scanners software and then import image to PS.
The download for the CS5 32 bit Twain plug-in is here: http://helpx.adobe.com/photoshop/kb/twain-plug-photoshop-cs4-cs5.html#main_Photoshop_CS5_a nd_TWAIN
If you can afford it, Vuescan does allow 32 and 64 bit scanning directly into Photoshop's 32 or 64 bit version. Visit Hamrick.com to get a demo.