To use a proxy on

, tools>options>advanced(the gear icon)>settings>manual configuration. *.*.*.* is the proxy and :* is the port(don't include ":"). Now, your IP is *.*.*.*, making you appear american.
On

click the wrench icon>options>under the botnet>lan settings>proxy. Rest is same as in Firefox.
Have in mind that many proxys are slow, others are simply not operating, and some even don't hide your IP(fakes). You may need to google more american proxies.