Add (jQuery) at the end of the plugin. Demo: http://jsfiddle.net/wLEEK/
Here's why that works. Right now, your plugin looks like this:
(function($) {
$.fn.myplugin = function(options) {
//some code.
options.callback();
}
})(jQuery);
You're creating the jQuery method myplugin. But notice the code on the first line function ($) { and on the last line }. You've wrapped the plugin creation code in a function. The function accepts one argument that you call $. So the plugin creation code is inside a function, so we call that function using (jQuery) - passing in jQuery as an argument. It's kinda complicated, but here's a more familiar form to help you understand what's going on:
function createPlugin ($) {
$.fn.myplugin = function(options) {
//some code.
options.callback();
}
}//end of createPlugin
// myplugin isn't created yet; we need to call the function createPlugin
createPlugin(jQuery) // now myplugin is created