Hi @lpmorgan1,
you can manually add the package using the following commands:
Install the repository GPG key (open a terminal with Ctrl + Alt + t and paste the following commands):
(First one then the other!).
wget -q -O - http://archive.getdeb.net/getdeb-archive.key | sudo apt-key add -
sudo sh -c 'echo "deb http://archive.getdeb.net/ubuntu xenial-getdeb apps" >> /etc/apt/sources.list.d/getdeb.list'
Update the package index:
sudo apt-get update
Install freefilesync deb package:
sudo apt-get install freefilesync
Hope it helps!. 
ATTENTION: USING THE ABOVE COMMANDS WILL INSTALL A 3RD PARTY PPA AND YOU NEED TO KNOW THAT IT WILL UPDATE OTHER PACKAGES ON YOUR SYSTEM THAT ARE OUTSIDE OF THE NORMAL UBUNTU REPOSITORIES!. (USE AT YOUR OWN RISK!).