Hi Bryon--
It depends on your doctype, but for both Docbook and DITA, <indexterm> takes the index terms as element content, not as an attribute value. In the Docbook sample document, if you use the standard Find/Replace dialog, and set "Find What" to "Arbortext" and "Find Within" to "indexterm", it will find the various indexterm entries containing "Arbortext".
If you are using a custom doctype where index terms are specified as attribute values, maybe you could post a sample of your indexterm markup.
(Of course, in your post you typed "indextern" as your tag name, but I assume that was just a typo when creating the question, and you typed it correctly in the search dialog.)
--Clay
