Hi im getting this error in my script Assets/MouseManager.cs(11,27): error CS1525: Unexpected symbol `Vector3'

using UnityEngine;
using System.Collections;

public class MouseManager : MonoBehaviour {

void Update() {
	if( Input.GetMouseButtonDown (0)
	    // We clicked, but on what?

    
	    Vector3 mouseWorldPos3D = Camera.main.ScreenToWorldPoint(Input.mousePosition);

	    Vector2 mousePos2D = new Vector2(Input.mouseWorldPos3D.x, Input.mouseWorldPos3D.y);
	
	    Vector2 dir = Vector2.zero;

	    RaycastHit2D hit = Physics2D.Raycast (mousePos2D, dir);
	    if(hit != null && hit.collider!=null) {
	       // We clicked on SOMETHING that has a collider
	       if(hit.collider.rigidbody2D !=null) {
	           hit.collider.rigidbody2D.gravityScale = 1;
		  }
	  }
  }

}

You need brackets over

 Vector3 mouseWorldPos3D = Camera.main.ScreenToWorldPoint(Input.mousePosition);
         Vector2 mousePos2D = new Vector2(Input.mouseWorldPos3D.x, Input.mouseWorldPos3D.y);

Like this:
{

}

Your if is missing the closing parentheses, and the opening brace…

if( Input.GetMouseButtonDown (0) )   // <-- add a )
{      // <-- add {
   ... the rest of the stuff ....

}  // <-- add this at the end

You should end up with this:

using UnityEngine;
using System.Collections;

public class MouseManager : MonoBehaviour
{
  void Update()
  {
    if(Input.GetMouseButtonDown(0))
    {
      // We clicked, but on what?
      Vector3 mouseWorldPos3D = Camera.main.ScreenToWorldPoint(Input.mousePosition);
      Vector2 mousePos2D = new Vector2(Input.mouseWorldPos3D.x, Input.mouseWorldPos3D.y);
   
      Vector2 dir = Vector2.zero;
      RaycastHit2D hit = Physics2D.Raycast (mousePos2D, dir);
      if(hit != null && hit.collider!=null)
      {
        // We clicked on SOMETHING that has a collider
        if(hit.collider.rigidbody2D !=null)
        {
          hit.collider.rigidbody2D.gravityScale = 1;
        }
      }
    }
  }
}

Pay close attention to how you format and indent things - make sure the formatting is consistent and the braces/brackets line up. It really helps identifying missing things.