You need to create the repository file with sudo as well.
Try this:
$ echo "[MongoDB]
name=MongoDB Repository
baseurl=http://downloads-distro.mongodb.org/repo/redhat/os/x86_64
gpgcheck=0
enabled=1" | sudo tee -a /etc/yum.repos.d/10gen.repo
$ sudo yum update
And then:
sudo yum install mongo-10gen mongo-10gen-server
Hope it helps