I got one working connected to a friends R-Pi (not running 16.04 though). These were the steps (from my notes):
Open a terminal and enter the following commands
sudo bluetoothctl -a
power on
agent KeyboardOnly
default-agent
pairable on
Enter discovery mode on your keyboard
scan on
You should see your keyboard appear on the list. Copy its MAC address (xx:xx:xx:xx:xx:xx) to the clipboard.
Now type
pair MAC_ADDRESS
trust MAC_ADDRESS
connect MAC_ADDRESS
where MAC_ADDRESS is the address you copied to the clipboard.
You should see something like:
[bluetooth]# connect xx:xx:xx:xx:xx:xx
Attempting to connect to xx:xx:xx:xx:xx:xx
[CHG] Device xx:xx:xx:xx:xx:xx Connected: yes
Connection successful
Now type:
quit
I can't remember where I found the guide, but hopefully that will work for you too.