add_action(‘wp_ajax_[action name]’, myfunction) problem

In my projects I do it like that

PHP

function foo() {
    echo 'bar';
}
add_action('wp_ajax_foo', 'foo' ); // executed when logged in
add_action('wp_ajax_nopriv_foo', 'foo' ); // executed when logged out

Javascript

data = { action: 'foo', avalue: 'some value', 'anothervalue': 'another value' };
jQuery.post(ajaxurl, data, function(response){
    alert(response);
});