If you're getting the error message: "Modifying CST Folder requires an administrator name and password" that just means you don't have the channel strips folder on your computer. That step in the installer is intended for people who want to use the channel strips in Logic- but won't really have any impact on using it with MainStage.
We will actually be removing that step from the installer in the near future, so I would just skip it.
If you want to create the folder you can in your Audio Music Apps folder and call it Channel Strip Settings.