MongoDB 3.0.x (current stable version) doesn't have an official repo for Fedora 21 and/or Fedora 22 but you can install it by using the CentOS/RHEL repositories. To do this, follow these instructions:
NOTE: There are only x86_64 builds.
1) Add the repo
$ su -
# touch /etc/yum.repos.d/mongodb.repo
# nano /etc/yum.repos.d/mongodb.repo
And for Fedora Workstation add the following code to the file:
While for Fedora Server these are the needed lines:
2) Install MongoDB
sudo dnf -y install gperftools-libs mongodb-org
systemctl restart mongod.service
systemctl enable mongod.service
3) Check everything works
on the command line should prompt you with the mongo shell and visiting localhost:27017 in your browser
Should return what you see on the image above, which means everything it's working... The MongoDB shell might prompt you with some warnings about tips for better performance, you can always apply them to your system so your setup works better (especially on production enviroments).