Use cases can only truly be associated with requirements in documents, not the documents themselves. To associate a use case with a requirement in a new Rational RequisitePro document, you must have "Create" permission for that document type in RequisitePro. See your RequisitePro project administrator for appropriate access.
Select a new requirement to be association and the document type of the document you want to contain that requirement. RequisitePro creates and opens a new document of the document type specified by the Default Document Type box in the Associate Model dialog box. RequisitePro also creates a requirement in that document and associates it to the use case. The name of the use case is used to create the new document name and file name.