You can install the TextMesh Pro add-on via the Setup Wizard.

If this doesn't work for some reason, you can install it manually instead:

  1. Go to the packages folder at Assets/TheraBytes/BetterUI/Packages
  2. Import the package BetterUI_TextMeshPro_EditorPanelUI
  3. If you see compile errors, import the package BetterUI_TextMeshPro_UiEditorPanel

After installation also TextMesh Pro components can be made Better.