A lot depends on what kind of project it is. WTP's JSP support either expects the JSP files to be under the same folder that's the parent of the WEB-INF folder (src/web, which it will then treat as "/" to find TLDs), or to have project metadata set up to help it know where that root is (done for you in a Dynamic Web Project through Deployment Assembly). How are you referring to the TLD file, and where is the JSP file located?
And maybe I missed the original post to the Eclipse forums; the one I saw was posted a full day after this one.