I think the problem you're seeing is from no JSCLASS_PATH being defined. I remember this being a bit mysterious when I first got JSClass working. Here is a minimal html doc that will load JS.Class for you and calls an init function. It assumes JSClass is in the directory ./src/js/jsclass relative to the HTML file.
<!doctype html><html><head>
<title>Minimal JSClass Example</title>
<script>
var global = window; // Node.js compatibility.
global.ROOT = global.ROOT || './src/js/';
JSCLASS_PATH = global.ROOT + 'jsclass/min';
</script>
<script src="./src/js/jsclass/min/loader-browser.js"></script>
<script>
JS.Packages(function() {with(this) {
file(global.ROOT + 'jsclass/src/core.js').provides('JS.Class','JS.Module','JS.Singleton');
}});
</script>
</head><body>
<script>
JS.require('JS.Class', function() {this.init();});
function init() {
console.log('init called');
}
</script>
</body></html>
I use JSClass in a web framework I work on so you might be able to get some relevant things out of that. It's on github at
https://github.com/maynarddemmon/myt