To expand a little on what Terera said, in Mobipocket Creator, go to the "Guide" tab, and fill it in as follows:
Title: Table of Contents (this is what will appear on the "Goto" menu in Mobi Reader).
Type: toc (select this from the drop-down list)
Filename: book.html#toc
This last entry is the hyperlink to your table of contents. If it's a separate file, it can just be a filename; if it's a page within your book, you'd mark it in the HTML with:
<A name=toc></A>
and then, as shown, reference it as "book.html#toc". You can of course use whatever name you wish; I use "toc" since it matches the guide type.
Once you've filled in the page, don't forget to click the "Update" button at the bottom to write the changes to your OPF file.
Now, after you've built your book, you should have an active "Table of Contents" link. On the Windows Mobi Reader, this will be an item on the "Goto" menu. On other devices, there will be a dedicated "Table of Contents" button or menu item. Either way, it should now be "active".
While you're editing your "Guide" page, you may wish to add a cover image for your book. To do this, fill in another line as follows:
Title: Cover Image
Type: other.ms-coverimage
Filename: cover.jpg (or whatever your cover image is called)
Make sure that the cover image file is in the same folder as the OPF file. This cover image will now show up in the "Library" view of any Mobi reader and, once again, if your reading device has a navigation link to display the cover image, it should be active.
Important note
If you add a cover image,
do not include it on the "Publication Files" page of Mobi Creator (this applies to all other illustrations in the book, too). If you do, the image will be included in the file
twice.