I had to read this post several times before I caught on that the error message is being displayed by Eclipse. I must be getting senile in my old age. ;-)
You need to add the tag library, or the jar file containing the tag library, to your portlet's Eclipse project. That specific tag library is located in jboss-portal.sar/lib/portal-portlet-lib.jar. After you add this jar file to your project, you might have to close and reopen the jsp file to get the jsp editor to realize that it now has access to the tld file.
Thanks very much!
All The best!