It's very possible the browser hasn't finished downloading munchkin.js when you make the call to mktoMunchkin().
You could use jQuery to load muchkin.js.
$.getScript('http://munchkin.marketo.net/munchkin.js', function() {
//The code inside this anonymous function is executed by $.getScript() when the script has finished
//downloading.It is called a Callback function. It is required because
//getScript() does not block and will return before the javascript file is
//downloaded by the client
//If the call to getScript was blocking, the client would be frozen until the
//js file was downloaded, which could be seconds and could lead the user
//to think the browser has crashed
alert('Muchkin loaded. We can now use the Munchkin library.');
mktoMunchkin("476-IFP-265");
});
//any code placed here will execute immediately. When this code is executed,
// munchkin.js may not have finished downloading. Hopefully you can see why
//there is a need for the callback function in $.getScript().
This way you are guaranteed munchkin.js is fully downloaded before trying to use it's functions.