Make sure you have installed the virtualbox guest additions package on the host and then install guest additions on the guest.
First install the guest additions package on the Ubuntu host:
sudo apt-get install virtualbox-guest-additions-iso
Then open virtualbox, boot into RHEL and follow these steps (source):
Install DKMS, a framework that helps with building and upgrading kernel modules and the GNU C compiler (as root):
yum install dkms
yum install gcc
Mount the guest additions installation .iso (I have a windows guest but the VBox menus will be the same):
Install guest additions (again as root):
Modules are the linux equivalent of windows' drivers. The command below will load the necessary module (if it is not there) for mounting the shared folder (as root):
Try mounting the share now:
mount -t vboxsf Desktop /media/ubuntushare
This, of course, assumes that you have set up the share correctly.