Here's how they create a hitArea/hitRadius in the /assets/SpaceRock.js file in the examples folder.

gistfile1.js

  p.hitPoint = function (tX, tY) {
		return this.hitRadius(tX, tY, 0);
	}

	p.hitRadius = function (tX, tY, tHit) {
		//early returns speed it up
		if (tX - tHit > this.x + this.hit) {
			return;
		}
		if (tX + tHit < this.x - this.hit) {
			return;
		}

		if (tY - tHit > this.y + this.hit) {
			return;
		}

		if (tY + tHit < this.y - this.hit) {
			return;
		}

		//now do the circle distance test
		return this.hit + tHit > Math.sqrt(Math.pow(Math.abs(this.x - tX), 2) + Math.pow(Math.abs(this.y - tY), 2));
	}