The way I do it is to put a tag on my player called "Player" and in my PlayMaker action I use "Find GameObject with Tag" and select the "Player" tag. Now that PlayMaker has found the GameObject using the player's tag, you can store the GameObject in a GameObject variable. Then, add a "Move Towards" action after the "Find GameObject with Tag" action and select the GameObject variable you just created. This will make your zombie find the player by tag, store the variable and then follow that variable around the level.
