Doris, it's been a long time since I played this game, but I looked up this part in the "Official GK3 Stategy Guide". It says, after the circle appears on the map:
1. Click on the edge of the circle - it darkens and the cursor changes.
2. Size the circle to fit the points entered. To do this, move the cursor over the edge of the circle, click and hold, and drag the circle to make it bigger.
3. Move the circle over the points. to do this, move the cursor inside the circle and drag the circle to move it.
4.When the circle is in the correct position, the circle locks in place and SIDNEY reports the coordinates at the circle's center. A note with these coordinates now appears in your inventory.
Hope this helps. I loved playing this game.