If your installation is working, you probably have a Tor-proxy running as a service in the background. By default, Tor SOCKS port is 9050, so normally all you have to do is configure your client to use as SOCKS proxy "localhost:9050".
Attention though: it's important that your client doesn't leak your identity through the communication, or by opening a listening port. Satoshi's client is designed to support Tor, so the devs take the necessary care for it not to happen. I suppose MultiBit's developers do the same, but you'd better ask them to be sure. (btw, that's why the tor people release a browser bundle, to make sure the browser does not leak your identity)