The spokes were not stressed during the wheel building process. The spokes are stretching which is the reason for the rim going out of true. This is a common problem on cheap wheels. Your LBS might true it for you free, as they should have pre-stressed it and trued it before you purchased it. Then again they may charge you for that service if they like. This might be a good time to learn how to do it yourself. I would advise dropping by your local thrift store and purchasing a cheap bicycle to practice truing it's wheels before putting that spoke wrench on your good wheel!