one method
for anyone who this will help, what you want to do is use the path tool. you can find this in the toolbox.
decide where you want a diameter of your circle to be.
click once for one endpoint of the diameter.
then click again for the other endpoint of the diameter.
this should create a line segment.
click on the line segment and continue to hold down while you drag it to your desired position. an arc should begin to form and move with your cursor.
finally, stroke the path using your desired settings (pixel size, color, etc.).