If you want to add a Sitemeter widget to WordPress sidebar you can use this plugin. True, you can just paste the sitemeter 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 Sitemeter widget here (just change the .txt extension to .php).
Drop the sitemeter.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 Sitemeter counter code.