If the Megascans button is not visible on the Editor Shelf even after successfully installing the plugin, then it could be due to a missing library in your Editor. This library is PYMEL for the Python 3 interpreter of your Maya 2022 Editor.

Probable Cause of the Missing Library

Starting with Maya 2022, Autodesk is transitioning Python support from py2 to py3.

Maya 2022 comes compatible with both py2 and py3 python interpreters. However, it initiates with py3 python-mode by default.

Both the Python interpreters have a library called PYMEL which is required for the installation of the MS Plugin in the Maya Editor.

The installation of this library varies depending on the python version:

• py2 always gets installed (by default) with your Maya 2022 Editor installation.
• py3 only gets installed when you select the component ("PYMEL for PYTHON3") during the installation process of Maya 2022 Editor (as shown in the image below).

It's possible that you did not enable the PYMEL for Python 3 checkbox on the installation wizard.

You can confirm that PYMEL is missing from your editor by following these steps:

• Open the Terminal app on your PC (CMD + Space and type Terminal).
• Once open, you will see the following window.

💡 Important Note: In your case, the system-name (as indicated by the red highlight below) and the username (as indicated by the blue highlight below) might be different.

• Go to the Maya 2022 root (bin) directory by writing the following command (as indicated by the red highlight below)and press Enter:
  cd /Applications/Autodesk/maya2022/Maya.app/Contents/bin/

The terminal will now point to the said directory (as highlighted below).  

• You can double check if you're in the correct directory by writing pwd in the Terminal and pressing Enter. This will show the complete path of the directory you're currently in (as indicated by the blue highlight below).

• Now type the following command to check if the PYMEL library (for Python3 interpreter of your Maya 2022 Editor) is installed or not (as shown by the red highlight below). 

"sudo ./mayapy -m pip list"

💡 Important Note: It is very important that you run the above-stated command with Admin rights (i.e. using "sudo"). Otherwise, you won't be able to utilize the mayapy command.

• Since you will have invoked admin rights (via the sudo command), the terminal might prompt you for the admin password (as shown in the highlight below). 
  Enter the password and press Enter.  (Your password keystrokes might not show while entering so don't get confused)

• Now all installed Python libraries will be listed on the Terminal. 
  If installed, the PYMEL library will be listed in the results (as highlighted below). 

• If it's not installed, the PYMEL entry (as indicated by the red highlight below) will NOT show. And that is what's causing the issue here. 

Solution

• Close Maya Editor (if running).
• Head back to the Terminal (reopen it if it was closed).
• Make sure that you are in the (root i.e. bin)directory of your Maya 2022 Editor (as shown by the red highlights below).
  You can confirm using the pwd command as well.

• Once in the root (bin) directory, type the following command and press Enter (as shown in the red highlight below):

sudo ./mayapy -m pip install "pymel>=1.2.,<1.3." 

This will download and install the latest version of the PYMEL library on your machine (as shown in the blue highlight below).

• Once successfully installed, type the following command (as shown in the red highlight below) to confirm if the PYMEL library (for Python3 interpreter of your Maya 2022 Editor) was installed.

"sudo ./mayapy -m pip list"

Now, you should see the PYMEL entry in the results (as indicated by the red arrow below).

• Next, reinstall the MS plugin (in case, you had uninstalled it) and start your Maya 2022 Editor. The MS Plugin should appear in your Editor shelf now.

💡 Important Note: Make sure to select "Allow" for the 2 prompts that might appear upon startup regarding the MS Plugin.