Featured image on twentyten theme

Hi,
I’m using the “twentyten” theme but can’t figure out how to enable the ” featured image ” .
For example , I create a new post/ page and insert an image in the “Featured image box ” .
When the page / post is accessed through my browser , the header image remains the default ( it supposed to show the featured image ) .
Do I have to make extra configurations on the Dashboard to enable the “featured image” feature ??
Thank ….