As this a taglib rather than a tag, you'll need to specify it in web.xml - something like:-
... just substitute in the filename your stored your taglib under for 'tagfile.tld'
or alternatively, put the tld into WEB-INF/lib directory as a JAR.
Remember that the URI is just an identifier, not a physical location, so you need to provide the mapping between the URI and physical location in some way, otherwise the compiler won't know what to do!