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"
/>