You can password protect your web statistics directories in both Plesk 5 and Plesk 7.5 Reloaded.
In the Plesk 5 control panel there is a button labeled "directories" which allows you to set up password-protected directories.
In order to password protect your web stats directory, you'll want to choose "non-SSL" for the protection method, then when you're asked to specify the directory, just enter /webstat in the provided field.
You can now create as many users and passwords as you'd like for that directory.
Password protecting the webstats directory is a built-in feature for Plesk 7.5. Reloaded. Click on the Setup button for the domain for which you wish to password protect the web statistics. One the "Web Statistics" line, check the box to the right of "accessible via password protected directory '/plesk-stat/'". Use your domain FTP username and password to access your web statistics at https://mydomain.com/plesk-stat/, replacing "mydomain.com" with your actual domain name.