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.list

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