You need to add the "TOC" entry in the GUIDE section of the OPF in order for the TOC option to be made available (see below).
Code:
<guide>
<reference href="Text/Frontmatter.html#coverpage" type="cover"/>
<reference href="Text/Frontmatter.html#coverpage" title="Start here" type="other.start"/>
<reference href="Text/Frontmatter.html#TOC" title="Table of Contents" type="toc"/>
</guide>
Friendly reminder when you do the conversion from epub to mobi via Kindlegen , it uses the TOC you declared in the OPF not the entries in the NCX so it usually makes sense that your HTML TOC and NCX contain the same entries.