Old topic, but to keep this reply with the thread for the next person that searches...
It appears that "Active Tasks" are those for which you have started a timer. So if you select a task, and start a timer it should show up in the Active Tasks area. I say "should" because as I'm testing OpenGoo 1.6beta3 I see that there's a refresh problem where the data is not accurate unless you reload a page--it should refresh (pull new data) as you switch tabs.