The Endertainer rig comes in an Asset Library format. The Asset Library allows you to import rigs and other assets from the Asset Browser. To use the Asset Library, you need to install it first.
Note that if you want to install a newer version of the asset library from the same major version (for example, MCS3 v1.0 -> MCS3 v1.1), you need to uninstall the old one first! This is to prevent conflict with the older version.
Here's how to install the Asset Library:
Go to Edit -> Preferences. A window should pop up.
Go to Edit > Preferences to open the preferences window
Select File Paths and go to the Asset Librariessection.
Go to the Asset Libraries section in File Paths
Select the + icon on the right side of the asset library list to add a new path. Then select the folder icon on the Path field to assign the asset library path.
Select the + icon to add a new Asset Library path
Look for the folder containing the asset library. Double-click on the folder and then select Add Asset Library.
Double-click on the asset library folder to open it
Select Add Asset Library to install the asset library
Change the Import Method to Append (not the one with Reuse Data).
Change the Import Method to Append or Link
Once you've done that, the Asset Library should now be installed. To import any assets from the Asset Library, check the Asset Browser Method on the Importing Assets page.