The wiki is not yet updated for downloading the F21 repo rpm.
https://wiki.trinitydesktop.org/FedoraInstall
I was able to figure out the location of the F21 R14 repo rpm.
The trinity web site redirect takes me to tde-mirror.yosemite.net, which seems fully populated. Unfortunately, the mirrors list does not include tde-mirror.yosemite.net and yum fails.
http://ppa.quickbuild.pearsoncomputing.net/trinity/trinity/rpm/f21 redirects to:
http://tde-mirror.yosemite.net/trinity/trinity/rpm/f21/
Mirror list:
http://tde-mirror.yosemite.net/trinity/trinity/rpm/f21/trinity-r14-x86_64.li...
None of the sites listed in the mirrors list have Fedora 21 R14 packages. If the mirrors list included tde-mirror.yosemite.net then yum would be able to download packages.
Darrell