Defining a circular cutout in a pad in Altium
I would do this by defining the outline and then creating a solid region from the outline. Eg. Snap grid set to 0.025mm. Set at 0,0 for center so that the center arc can be used to snap to the ends of the lines (do them first). Takes just a few seconds to draw this. 



Then Tools->Convert->Create Region from Selected Primitives



And then add a pad to the region etc.