If you want a semi-circle with a leader, I don't know of any other way than to sketch the semi-circle and then place the leader from a Note. Where you would normally enter text for the note, just hit the spacebar on your keyboard once or twice. That will give you a leader with no text box. If you don't do that you'll get the box with the X through it.
The end of the Note's leader should snap to the end of the semi-circle, and then you can grab the other end to shorten it and change the angle of the arrow so it looks right in relation to the semi-circle. You can set their colors, line thickness, etc. with Layers or the Line Format toolbar.
I hope this helps. If I misunderstood, and this isn't what you want, please explain further.