It used to be, in Joomla 1.5, that if you wanted to see the module positions you can either click on the Preview button in the Template editable area, or add ?tp=1 to the end of your URL to see the different module positions from the front-end of your website.

However, for security purposes,since Joomla 2.5, you must enable this option in the back-end first. If you are in the Template Manager> Template tab, you will see the Preview link is disabled.

To enable it:

It is recommended that after you are done with development, you disable it again.