Create a "Table of Contents" page in your book, containing hyperlinks to each of your chapters. At the start of the page, add an anchor with a name. This will be the point in the book that is jumped to when the table of contents is chosen:
(It doesn't matter what you call it, but "toc" is convenient.)
Now, open your OPF file in MobiPocket Creator, click the "Guide" tab, and enter a line like this:
Title: Table of Contents
where the last entry is the name of your book file, of course.
Don't forget to click the "Update" button at the bottom of the page.
Now, build your book, and you will (hopefully!) have a working table of contents link.