Set The AA Filtering option.

The AntiAliazing filter can be set to either 0,2,4 or 8. This coresponds to the number of multisamples used per pixel.

	// Set AntiAliasing to use 2x Multisampling
function Start() {
	QualitySettings.antiAliasing = 2;
}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Start() {
        QualitySettings.antiAliasing = 2;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Start() as void:
		QualitySettings.antiAliasing = 2

See Also: Quality Settings.