Try updating your video card drivers. It worked for me.
1) Location of Camera Raw Plugin
With PSE8, the file Camera Raw.8bi has been moved from
C:\Program Files\Adobe\Photoshop Elements 8.0\Plug-Ins\File Formats
C:\Program Files\Common Files\Adobe\Plug-Ins\Elements 8\File Formats
This is not documented in the ACR 5.5 Installation Guide yet, but PSE 8 comes with ACR 5.5 anyway.
2) ACR not loading, but PSE Editor
This sounds to me like the Screen Resolution Problem described here.
The problem was resolved by simply updating the video card device drivers.
Thanks to tech.support at onOne Software who suggested it might be the case.
This was the driver I had was NVIDIA GeForce 9800 GT Driver Date 02/08/2008 Driver version 18.104.22.16883
This is the driver I have now is NVIDIA GeForce 9800 GT Driver Date 27/09/2009Driver version 22.214.171.12407