I will explain this in a easier way.
First go to the Admin CP.
Next, click the Styles & Templates column, and select Style Manager.
Then, find your skin. (If you only have one skin, it would come up Default Style, or whatever you changed the name to).
Next, You should see these arrows, " << >> " to the right, click them.
Then, Scroll down until you see Postbit Templates >>.
Click that, and look to see if you can spot postbit_onlinestatus, if you do, click it, if you dont, it should be in white text under the thing you clicked.
Next, click edit.
Then, removed <img> and /user_offline and change it to Offline
For /user_online, change it to Online.
That's how.