You can put 2 instance variables in your world, called userX and userY. Update them from your player's object. This is what you basically need for that.
Your enemy needs to acces them, so create to methods:
public int userX()
{
return userX;
}
public int userY()
{
return userY;
}
I give you the advice to first learn more about programming. The greenfoot book is a great way to that. It's not a schame, but it's better to first understand the terms we're talking about here. After you know Greenfoot and java, you can come back.
http://www.amazon.com/Introduction-Programming-Greenfoot-Object-Oriented-Simulations/dp/0136037534
Good luck and I'm looking forward for your scenario's!