virtualbox.org

You must be missing something in the guest. I had no issue with guest additions in version 6.1 and none with versions up to and including 6.4

Read this Howto: Install Linux Guest Additions and use the example for Fedora. Also if this was a server install are you certain that you installed the additional packages for a desktop manager and it associated packages?

Ok.. I will do a fresh reinstallation of RHEL 6.1 again, with those that shown in the link.
However I do not clearly know what is the additional packages required for a desktop manager and guest additions.
Maybe you can advice me on this.
Thanks. Appreciate your helps.