[Brad]

Most likely onlinetoday.php would be uploaded to the forum directory, or the same directory that your showthread.php file is in.

You add phrases via your admin cp, in the navigation frame find 'Languages & Phrases', click on 'Phrase Manager'.

A new page will open up, at the bottom there should be a button labled 'add phrase', click it.

This will bring you to the a new screen, add the phrases like this:

Varname: last_online_at_x
Text: Last online at {1}