You will want to use a powered directional antenna like this one:http://www.newegg.com/Product/Product.aspx?Item=9SIA0SF0H19277
Your adapter uses a SMA type connector, so any powered directional antenna (pointed at the router) should help.
However, mooching off of a McDonald's network is not cool and could technically be considered as theft. If they notice you on the net all the time, they will likely block your access.
Something to think about.