Just installed VirtualBox on OS X and need the Extension Pack? This (really long) one-liner will download and install it for you. Run in terminal and enter your password once and you’re off to the races!
export version=$(/usr/bin/vboxmanage -v) && export var1=$(echo ${version} | cut -d 'r' -f 1) && export var2=$(echo ${version} | cut -d 'r' -f 2) && export file="Oracle_VM_VirtualBox_Extension_Pack-${var1}-${var2}.vbox-extpack" && curl --silent --location http://download.virtualbox.org/virtualbox/${var1}/${file} -o ~/Downloads/${file} && VBoxManage extpack install ~/Downloads/${file} --replace && rm ~/Downloads/${file} && unset version var1 var2 file
Note that VirtualBox must be installed first :)