First:

$ cd yum.repos.d
$ sudo wget http://download.virtualbox.org/virtualbox/rpm/fedora/virtualbox.repo

Contents:

[virtualbox]
name=Fedora $releasever - $basearch - VirtualBox
baseurl=http://download.virtualbox.org/virtualbox/rpm/fedora/$releasever/$basearch
enabled=1
gpgcheck=1
repo_gpgcheck=1
gpgkey=https://www.virtualbox.org/download/oracle_vbox.asc

Play it safe and update:

$ sudo dnf update

Install:

$ sudo dnf install VirtualBox-5.2

Make sure you're a member of the vboxusers group.

Click http://www.oracle.com/technetwork/server-storage/virtualbox/downloads/index.html#extpack, and install.