You would have to create a page like management.asp with links to each of the pages.
When they click a link you would pass the filename of the file. In this case aboutus.asp. This would take you to a page like editor.asp. Which loads the cutesoft editor with the page loaded into it. You would then create a save.asp page which would take the results of changes on the page and perform the editor.save(filename) to commit the changes to the orginal aboutus.asp page.
I hope this makes some sense. the editor itself would not be on the actual aboutus.asp page it would be on a management page. You could put a login page prior to the management page if you like a simple login.asp that reads the username and password and checks for validty and then redirects to the management page would suffice. Then using a server side include file you can check for a session variable to verify they have logged in otherwise redirect back to login.asp.