You could to check the distance. If Player and Enemy in some distance, enemy -> Follow Player.
First, you can Add event -> System -> Compare two values.
1. On First Value: distance(Player.X, Player.Y, Enemy.X, Enemy.Y)
2. Set the Comparison to less or equal.
3. On the Second value enter the distance you want.
Then add action to follow player. Hope this helps <img src="smileys/smiley2.gif" border="0" align="middle" />