In any SSH window, type the following
cd /etc/yum.repos.d/ wget http://centos.karan.org/kbsingh-CentOS-Extras.repo rpm --import http://centos.karan.org/RPM-GPG-KEY-karan.org.txt nano /etc/yum.repos.d/kbsingh-CentOS-Extras.repo
In the edit window, change both sections to include enabled=1.
enabled=1
At this point, you can install proftpd as follows
yum install proftpd
Remember to set enabled back to 0 after you're done, just to clean up.