Go to Appearances > Montezuma Options > Sub Templates. The postformat.php "virtual" file determines how the posts are formatted on your blog page. By default, it calls bfa_excerpt(), which unfortunately doesn't use the excerpt field. Change it to the_excerpt() and you'll get the contents of the excerpt field.
To see the different functions you can use in the virtual files, click the Limited PHP Code tab in the upper right corner.
If you have other questions related to the Montezuma theme, you can post them on the Bytes for All support forum.