Can you please check for existence of file "com.adobe.PDFAdminSettings.plist" at the following locations:
1. /Library/Preferences/
2. ~/Library/Preferences/
This file might be already present on your machine if you have any other version of Acrobat installed. You need to rename this file or move it to some other location and then try installing Acrobat 11 again. Please let us know if you are able to install after following these steps.
-Shashi