You had left out the "Import Video Frames To Layers" part, which is quite different from just "saving GIFs". That's a whole 'nother story.
In the Mac world, the 32 bit and 64 bit versions of CS5 are just one and the same binary.
You choose which version to launch by checking or unchecking the appropriate version in the Get Info dialog box of the application icon.
First, quit Photoshop. In the Finder select the application icon (NOT the folder in which it is contained !) and do a Get Info on it: Command+I:
