First, try this:
1) Go to ./Library/Application Support/Adobe/Plug-Ins/CS5/FileFormats
2) Remove whatever version of ACR is in there
4) Re-run the 6.7 installer
If that still doesn't work, then manually install the ACR 6.7 Plug-in in the location indicated under 1) above.
Make sure you start the path with your ROOT LEVEL Library, right after the name of your hard disk, not your user or home library.
Oh, and if you're running Lion (10.7.x) make sure Java runtime has been installed, as it is no longer part of the Lion default installation: