In the section of the documentation that you have referred to, specifically the section "Enabling RESTful Access to a Report Region" under the parent section "Implementing Web Services" in Chapter 17 of the Application Express User's Guide, you'll see the following note:
Note: The Enable RESTful Access List option only displays if RESTful access to this Oracle Application Express instance has been enabled, see "Enabling RESTful Access" in the Oracle Application Express Administration Guide.
Can you please confirm whether you've enabled RESTful access for your instance? If not, you'll need to do so before attempting to update your report settings. I hope this helps.
That's great you're up and running now. So now you've managed to expose your Report region as a RESTful Service. I think you might find it useful to read through the section Understanding Web Service References in the same chapter - http://docs.oracle.com/cd/E23903_01/doc/doc.41/e21674/advnc_web_services.htm#BABDCIBH - as it explains the various references that can be created. In your case, your Web Service reference is based on the RESTful style, and not on a Web Services Description Language (WSDL) document. If you read through the section Accessing a RESTful Enabled Report Region from a Web Service Client - http://docs.oracle.com/cd/E23903_01/doc/doc.41/e21674/advnc_web_services.htm#CHDEHFJI - the apex_rest service API can be used to discover available RESTful enabled reports for a given application. Its response is an XML document with a description of all reports that can be accessed by RESTful Web services.