RSA - Associate work item to design changes is disabled

By "cross server communication and association," do you mean you have friends and project level associations set up?

First, you need to add a friend server using the Friends tab in the Server Administration pages and authorize the request.  Then, in each project area you wish to link from, you need to set up associations by going to the Project Area editor and adding the artifact container.

For more details, please see "6. Learn how to set up CLM links" in http://www.ibm.com/developerworks/rational/library/25-server-client-tips/