There is no mirc command "/trace". Try asking the author(s) of the script or looking in the code. Also, trace requires a destination, as you cannot trace an unspecified path.
trace's behaviour varies from different servers, however it is designed to 'trace' the path to a specific nick or server (and show you delays in between each server), so its usually best used with those parameters (eg: /trace bunar or /trace services.server).
Many servers restrict this command to IRC operators now though.