If you want to add a Performancing metrics widget to WordPress sidebar you can use this plugin. True, you can just paste the pmetrics code into a text widget but when you are using WordPress MU that will not work. WordPress MU will not allow to paste Jave Script code into posts.
Get the Pmetrics widget here (just change the .txt extension to .php).
Drop the pmetrics-mu.php file into your wp-content/plugin and activate it. If you are running WordPress MU just drop it in the wp-content/plugins-mu directory.
Add the widget to your sidebar and enter your pmetrics number. The pmetrics number can be found in the first line of the java-script you get from the pmetrics web-site. It is the only number in the line.