Ok since this is Acrobat on Mac OS you will not be able to simply install the trial. Instead lets first try rebooting in safe boot and logged in as a Root user and try installing again. Please feel free to respond if you need additional details on how to complete either of these actions and I will locate the appropriate Apple technical document.
Ok and you did log in as root and saw Welcome to Mac OS X Safe boot?
If so we will try the longer solution which seems to have lead to success for many users. Please complete the following steps:
Please feel free to respond to this thread if you have any questions about any of these steps. I would recommend completing them in full as several users have reported success by following these directions.