There's a few ways of handling this depending on the specific behaviour you're after. Do you want the player to be able to move freely inside the circle, or only around the outside of the circle?
If the player can move freely within the circle its quite simple, use this event, just make sure to doublecheck the image point of the player character and collision polygon of the circle.
If you want to have the player only able to move around the outer edge, I would create a second smaller and invisible circle sprite, and place it inside the first circle, again making sure the collision polygon fits, and edit the event like this :
Hope that helps!
EDIT : For some odd reason the forum doesn't like my images, put links to them in instead.