best way to do cross-site request?

I am in a chrome extension background page. I want to query my CI app, cross-site for some info. What is the best way to do this?

XMLHttpRequest?
$.post?

some other way?

I tried using 

$.post("http://localhost/subit_backend/login_daemon.php","action=login", processResponse);

but the "$" is not found.

proof that an old dog can learn new tricks