Read the read me of that plugin. You're suppose to edit a language file.
Edit:
Here its what it says:
Quote:1) Open inc/languages/english/index.lang.php
2) Add:
$l['online_note_today'] = "{1} {2} active in the past {3} hours ({4} {5}, {6} of whom {7} invisible, and {8} {9}).";
$l['whos_online_today'] = "Who has been online today";
at the bottom of that file before ?>
3) Upload that file to inc/languages/english/index.lang.php
4) Upload onlinetoday.php to your inc/plugins folder.
5) Go and activate the plug-in in your admin CP -> Plugin Manager -> Show the users that has been online today
6) Enjoy.
You're welcome. And remember, always read the readme files!
They are there for a reason.