All of the sky is mapped by RA (around the polar axis in the hour, minute, second nomenclature) and Dec (from the north pole to the south pole). You also need to know the time of the sky (a slightly faster time than the earth's time - the sky turns fully around in about 365 days) and set the RA dial so that the mapped position of any object in the sky is read off of the RA dial. The Dec axis should be right if the pole is at 90deg. 0deg. is the equator.
As a result, you will go to a particular negative Dec. value and the correct RA (providing that you have the time set on the dial) and you should be seeing M42.