Microsoft Help Viewer 1.x requires extra meta elements for index terms, so converting DITA <indexterm> elements to HTML or XHTML is not enough.
To have DITA2Go prepare index entries for eventual use in Microsoft Help Viewer 1.x:
[Index] ; UseHVIndex = No (default) or Yes, prepare meta elements for use ; with Microsoft Help Viewer 1.x UseHVIndex = Yes
Index terms for Microsoft Help Viewer 1.x look like this:
<meta name="Microsoft.Help.Keywords" content="Marker, Plain index" /> <meta name="Microsoft.Help.Keywords" content="Unicode%2C mañana…" /> <meta name="Microsoft.Help.Keywords" content="Index marker, First" /> <meta name="Microsoft.Help.Keywords" content="Index marker, Second" />