Did the installer update your version of Photoshop or not? This installer will tell you if it doesn't find a compatible product.
When I saw "64-bit" I was afraid to continue and canceled the installation. I just tried it and it worked fine. I'm building a 64-bit box and look forward to installing the 64-bit version of CS5.
I liked the old system where all an ACR upgrade required was dragging one file (Camera Raw.8bi) into the proper folder.
Dragging one file was in the days of one-size-fits-all color, before camera-match profiles and custom camera profiles were possible.
Ken raises a good point here. It's the second time I've been caught out and cancelled it also.
The installer boldy announces itself as "Photoshop Camera Raw (64 bit) " and whilst this may only be a minor cosmetic issue it's misleading and to uninitiated somewhat confusing and annoying.
Needs a little tweak......Adobe?