display_user_activity function?

I am trying to use the display_user_activity function from the User Activity plugin.

In my theme header file, I entered:

<p>There are currently <?php display_user_activity(5); ?> users online.</p>

However, no value is ever displayed for <?php display_user_activity(5); ?>. The result is always:
"There are currently users online."

How can I use the display_user_activity function in my theme?