Not an expert, I just reversed engineered a working book
There are 2 entries in the EPUB2 books OPF
Code:
<spine toc="ncx">
...
<itemref idref="HTMLTOC"/> <<<links the filename assigned in the manifest
and a Guide entry for the inline TOC (if used)
Code:
<guide>
<reference type="toc" title="Table of Contents" href="Text/06_CONTENTS.html"/>