Open the OPF file in Mobi Creator. Go to the "Guide" page, and add an entry as follows:
Title: Table of Contents
Type: toc
Filename: mybook.html#toc
where the last entry is the address of wherever in your book you've put the TOC. In my case, I'd an an anchor in my book of the form:
<a name="toc"></a>
to mark the position.