post_thumbnail

if i put on home.php this code:

<?php
if (current_theme_supports(‘post_thumbnail’)) {
echo “OK”;
}
?>
the answer is NO!

how can i use post_thumbnail function? (on include/funtcions.php is written
add_theme_support(‘post-thumbnails’);
but seams doesnt work.

pls help me!!!!
tnks
dap