Have your end mill centred above the hole you want to drill at X0 Y0 Z0. Start the GCode, and it will cut a hole with the chosen diameter.
Tick the "sink" checkbox if you want to create a round sink, e.g. for housing a wheel that you want to finish off, so that all the inside material is taken out as well. If you don't tick the box, it's assumed that you mill all the way through the material, so that there is no need to remove the inside.