Adding shortcodes to text widgets in Simplemarket

Hi! I want to use a shortcode in my footer widget area for the plugin FormBuilder. I have added the code snippet

add_filter('widget_text', 'do_shortcode');

to my functions.php file. However, the shortcode is still not interpreted. Could this be theme specific? It seems to be working for everone else.

Thanks!

/Johannes Persson, Stilius