Where is the Templates Tab?
The Templates tab is provided as a way to edit your Custom skin (as opposed to any other skin). This is not the prefered way to edit, but it is provided as an option for people wanting to edit their skin online without having to re-upload their edited file(s).
If you use another skin and you would like to be able to edit it using the Templates tab you can edit the file /admin/b2template.php and replace the word "custom" with the skinname of your choosing. You then need to make sure that the skin you choose has the correct permissions set (CHMOD 766) on your server.
For more information about editing skins please see the section of the wiki entitled "Customize your blog".