In order for the "Go to Table of Contents" functionality to work. There must be a reference item in the Guide section of the OPF file identifying the html ToC.
Code:
<guide>
.
.
<reference href="Text/your_file_name.xhtml" title="Table Of Contents" type="toc" />
.
.
</guide>
The dots only indicate that there may be other reference tags before or after the one for the TOC.
Make sure the "href" attribute actually points to your html toc (correct file name/location). If your ToC is a single stand-alone html file, that's all you need to do. But if it's just a section of another HTML file, you'll have to identify it with an anchor (or any tag with an "id" attribute actually) and include that url fragment in the <reference /> tag's href attribute. In the HTML file:
and in the OPF file:
Code:
<reference href="Text/your_file_name.xhtml#toc" title="Table Of Contents" type="toc" />