Where do I put code to use hooks and events from Squarebit's 'Charge' plugin?
You'd listen to Charge's onCharge event just like you'd listen to any of Craft's own events, using a plugin.

Your plugin's init() method, for example could do this:

craft()-&gt;on('charge.onCharge', function(Event $event) {
    // ...
});


There's a nifty Craft plugin scaffold generator to get you started here, too.