Yes as root you have to edit the /etc/yum.conf
To open it you need to do from the command line a
vi /etc/yum.conf
Once in the vi editor go to the bottom of the file by pressing shift+G
Then hit the i to get into edit mode and paste the lines from my previous post. Once they have been pasted hit the esc key to get back to command mode then do a :wq! to save the file and exit.
If you don't want to use vi because it can be hard to grasp at first. I would suggest using pico and to edit the file with pico you should be able to do a pico /etc/yum.conf I don't have access to pico right now to send you the sequence you need to update the file but I think you might be able to figure out how to do it using pico's help.
If you don't know what vi is or are not comfortable with it. I would suggest this site as a place to start:
http://www.eng.hawaii.edu/Tutor/vi.htmlIf you are in the GUI of centos gedit or kedit should give you the ability to edit the file.