To get the opportunity browse again to a dtd, then you need to delete the cache directory, which is where Arbortext Editor stores the location of the browsed dtd. See Help > Session > Session (tab) and scroll down to "Cache directory".
Close Arbortext Editor. Use Windows Explorer to locate the Arbortext Editor's cache directory. Delete the doctypes directory in that cache directory.
Another method for placing your document types, stylesheets and related doctype documents is to place them in the ArbortextInstallPath\custom\doctypes directory. Arbortext Editor automatically checks this area for dtds. For further details see the Arbortext Editor Help Center for information about the custom directory.