Andy,
For the construction circle in sketcher:
1. draw a circle
2. pick the circle with the left mouse button (it should highlight red)
3. hit the right mouse button and select TOGGLE CONSTRUCTION
For the circle on a 2d drawing:
1. select SKETCH, CIRCLE
2. pick a point on the screen and begin the circle
3. while you are in drag mode with the circle, right click and select SPECIFY RADIUS
Good luck
JW