This article is geared towards Mac OS. For Windows, please visit this link.
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The process to set up the UE4 plugin is exactly the same as it is in Windows, except for the path where you have to install the plugin.

As the Installation folder must be the Plugins folder within the UE4 directory, in Mac, it is Macintosh HD\Users\Shared\Epic Games\UE_4.2x\Engine\Plugins 

This location may be different based on the way your system is set up. If you do not have the above location on your machine, try \Applications\Epic Games\UE_4.2x\Engine\Plugins

An example screenshot is shown below.

How to install the Plugin via Export settings

• Close Unreal Engine (if running).
• Head to your Local library tab as shown in the screenshot, and open Export settings. To access the Export settings, click the gear icon on the bottom left corner of the asset’s right panel in your "Local" tab, or go to Edit > Export Settings.

• Click Export Settings in the pop-up menu to view all the export options. You can tweak your settings, like texture format, LODs, mesh format (FBX or OBJ), etc. These settings are Global. 

• Head to Export Target, and enter the default path for installing the Mac plugin, i.e. Users\Shared\Epic Games\UE_4.2x\Engine\Plugins.
  
  
  
  
  
  
• The MS Plugin for Unreal Engine places all the Megascans assets inside a folder called Megascans in your Unreal project’s Content folder. If you want to customize this location, you can do so by specifying that location in the Default Project field. You may also leave it blank. 
  
  
• Now open UE4 and ensure that you're running the version you selected in Settings, and not any other. 

At this point, the Plugin button is visible now in the case of Windows OS. On Mac OS, it needs to be manually enabled. 

- Go to Edit > Plugins.

- In the Built-In section within the Plugins window, locate the Megascans plugin and check the Enabled box (as shown below). 

- Once you have enabled the checkbox, restarting Unreal Engine is necessary. Close Unreal Engine and open it again. Then the MS plugin button will be visible (as shown below in red highlight). 

- Click on the Megascans button to start the Plugin, and you'll be faced with the following:

Exporting an asset

First, start UE4 in the background. Open Bridge and select any downloaded asset to export. In the Export Settings, you may keep the Project Folder blank. Simply hit Export and you should see the following pop-ups:

The exported asset will start to appear in UE4.2x, as shown below.

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Should you face any errors at any point, please reach out to Quixel Support at support@quixel.com with your log file and details of your issue, by following the guidelines given here:

GUIDE: Creating a proper Support request

GUIDE: Basic Troubleshooting and requisite info before reporting