How to get enemies to slowly approach first person controller

Take a look at BurgZerd Arcade's Hack 'n' Slash tutorial series. One of the first videos has exactly what you need.

http://www.burgzergarcade.com/tutorial/unity-game-engine-tutorial-hs2-001

Without an example of your current code and showing us where you're having troubles, the best we can practically do is show you a tutorial (like Cherno did above) and give you a bit of pseudo code.

 private Vector3 _direction;
 // create a new Vector3 for the direction we need to go to get to the player.

 public float movementSpeed = 10.0f;
 // this will be how fast your enemy moves toward the player.


 void Update(){

 _direction = playerObject.transform.position - transform.position;
 // calculate the direction. thankfully, it's very simple
 // within the unity engine by simply subtracting your enemy's
 // position from the player's position.

 transform.Translate(_direction, movementSpeed * Time.deltaTime);
 // this does the actual movement. multiply movementSpeed 
 // by deltaTime to make movement framerate independent.
 }