Check to see if you have at least service pack 1 installed for Word 2008. From Word 2008, go to the Word menu>About Word. The splash screen should indicate Version 12.1.0 or greater. If you have 12.0.0 you will not be able to install the EndNote CWYW add-in.
Click on the following link to the Microsoft Mactopia web site: http://www.microsoft.com/mac/downloads.mspx
Scroll to the bottom of the page. Under Products click on Office 2008:

Then under the Downloads section click on Microsoft Office 2008 for Mac Service Pack 1 (12.1.0):

Under Details, scroll down and select the appropriate language (.dmg) file.

Install service pack 1 for Word 2008. Confirm that you have the service pack correctly installed by opening Word 2008, selecting the Word menu>About Word and the splash screen should now show Version 12.1.0.