Do you have the Option to Check In and Out files set for the site?
If so, you will need to checkout the webassist/security_assist/helpergrouprules.php file before making edits the access rules.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.