I had a Demo before would like your free version.
You need to go to their website and register for a free license at http://www.emclient.com/free-license. They will then email you a license key. Be aware, however, that if your demo period has expired, you will not be able to receive emails through eM Client and you must use another client or the webmail app to retrieve the license key.
After receiving the license, go to menu/help/license and enter the info.