The file server_processlist.php is missing in 3.5.5 (and 3.5.6), along with the associated tab. However you can still set this as the default tab in config, which then produces an error. Is this feature meant to be gone? I couldn't find anything in the documentation to say so.
In 3.5.x the feature has been moved under Status > Server, but you're right, the references to the old script are misleading.