Hi,
As far as I know, registration is not required for Office 2008 for Mac. This issue (the Office Setup Assistant opens when you try to open an Office application) usually occurs when the Office Setup Assistant detects any corruption in the license
file (Product Key) was used to install Office 2008 for Mac.
You might want to reload the software and reenter the produce key that is included with your original installation disk, then try again.
Also, please note this is the forum to discuss questions and feedback for Windows-based Microsoft Office client. Since your query is directly related to Office for mac, I would suggest you to post in the forum of Office for Mac, where you can get more experienced
responses:
http://answers.microsoft.com/en-us/mac/forum/macoffice2011?tab=Threads
The reason why we recommend posting appropriately is you will get the most qualified pool of respondents, and other partners who read the forums regularly can either share their knowledge or learn from your interaction with us. Thank you for your understanding.
Regards,
Ethan Hua
TechNet Community Support