Well, this a mel script and it works differently from plugins. In general, mel scripts go in your scripts directory (generally in My Documents -> Maya -> 4.5) and they are executed from Maya's command line or the script editor. Plugins get dropped in your plugins directory inside the main Maya program directory, and then you have to load them via the plugin manager preferences panel.
For this particular script, drop it in your scripts directory, and execute the script from within Maya (type toggleTriCountHUD in the command line and press Enter). What I've done is make a shelf button that executes this command. Most of the time, the name of the mel script file is the command you need to execute. Best thing to do is open the script in a text editor and read the comments.