I rewrote your plugin to make it more concise, but also to give it a function name which can be called from a template. I’ve turned it into a real activate/deactivate plugin. I’ve shrunk the script down to a single file, as well. You can check out the updated function here: https://conhuevos.com/uploads/online-status.txt
You know, I’m going to be nitpicky, but perhaps the left hand side could use a drop shadow. The left pane drops into the right, but there is no differentiation between the height of the left grey area and the right grey area. I’m sorry, is that asking too much? ??
I don’t understand what you mean by “write Page,” but I think you’re looking for is the “archives.php” page. Just edit that page to look like your “index.php” page and your visitors won’t be able to tell the difference. Sorry if this isn’t helpful.
