Use the Block Arc with Adjustment 2 set to 0. The following are step-by-step instructions:

1. Draw a Block Arc on the slide. The Block Arc an be accessed by selecting AutoShapes | Basic Shapes | Block Arc autoshape.
   
   
    
2. Set Adjustment 2 to 0 through ProTools Shape Adjustments toolbar.
   
   
    
3. This would get a nice top-half of a circle. You can rotate the half circle to get the half of the circle to any angle.