I edited the OP. Will do it again here for clearity.
Backup your sources.list file
cp /etc/apt/sources.list /etc/apt/sources.list.bak
Add a source to the sources.list
echo deb http://ftp.us.debian.org/debian/ jessie main >> /etc/apt/sources.list
Add another source
echo deb http://ftp.us.debian.org/debian/ wheezy non-free >> /etc/apt/sources.list
Update your package lists
apt-get update
Install the required programs…Type y when asked if you would like to proceed with the installation and hit enter
apt-get install par2 unrar
And finally let’s revert the changes made to sources list
mv -f /etc/apt/sources.list.bak /etc/apt/sources.list
Credits: http://community.wd.com/t5/WD-My-Cloud/GUIDE-NZBGet-11-0/td-p/619113
