Download this keynote example.
http://www.setfreedenver.org/trassh/halfcircle.zip
1. Draw a circle.
2. Select the circle and copy.
3. Open Preview... under the file menu... New from Clipboard...this will copy the circle into Preview. Do a SAVE AS (pdf).
4. Import the circle pdf into keynote. Apply a mask to the circle. Doing this allows you to use a mask on the circle. Make a half circle with the mask.
5. Make an arrow...shorten the staff of the arrow. Rotate the end of the arrow to match the circle end. Select all and group.
If you have Photoshop, Illustrator or Freehand you can do this in one step.