It depends how you installed it.
If you downloaded the tarball and extracted it then it's wherever you extracted it.
If you used a package manager to install it then they could be anywhere - you should look at the package manager software fedora uses (yum? it's been a while) to see how to find what files a package installs.
If you are running a modern ...
First you will need the arduino 1.6.x package from updates, not the 1.0.x package from fedora. Then:
You will be adding a URL to the new Additional Boards Manager URLs option: https://github.com/wasdpkj/Microduino-IDE-Support/raw/master/package_Microduino_index.json
This tutorial tells you where and how to add that URL to the preferences, as well ...
There seem to be two issues:
Getting the IDE to fully function when launched from the command
Getting the IDE to fully function (i.e. compile) when launched from
the Fedora GUI menu.
I have found a solution if I run the Arduino IDE from the command line, by adding the path to one of the cc1plus binaries to the PATH environment variable: