ACR is not expected to generate preview thumbnails in Windows Explorer.
You will not see thumbnails nor be able to preview your .CR2 files on a 64 bit system without the installation of a special codec to do the job. If you haven't already noticed, you can't even normally see thumbnails for .PSD files either (and this, at least, Adobe really should provide).
I use and recommend the FastPictureViewer Codec Pack, which will enable .CR2 thumbnails as well as (with just this week's release) .PSD thumbnails. It's inexpensive, and helps integrate your digital photography into your computer environment. I have no affiliation with this product, other than being a satisfied user.
Oops, I missed, on my first reading, that you are using an iMac. The above information is for PC only. Sorry about that.
The bridge version that comes with pse8 is cs4 bridge and can't use the 6.2 camera raw plugin.
The latest camera raw plugin for bridge cs4 is 5.7 which should read the canon EOS 7d files.
The pse 8 editor can use the 6.2 camera raw plugin, but you need to manually
update the plugin for the pse8 editor.
To update bridge to the proper camera raw plugin, just
use the Help>Updates from bridge and bridge should update itself.