You need to add a file fiware.repo in /etc/yum.repos.d/ with the following lines:
[fiware]
name=Fiware Repository
baseurl=http://repositories.lab.fiware.org/repo/rpm/$releasever
gpgcheck=0
enabled=1
I hope it helps you.
EDIT: originally, baseurl were http://repositories.lab.fiware.org/repo/rpm/x86_64/. That is still working, but the correct one since April 2016 uses $releasever. In addition, /x86_64 has been removed.
EDIT: originally, the name was repositories.testbed.fiware.org but it changed to repositories.lab.fiware.org on Feburary 2016.