it worked! thank you!
but why did it work? 
Repet:
it worked! thank you!
but why did it work?
Time to look at some basic electricity tutorials on the internet.
Repet:
but why did it work?
See this:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html
oh. I did not thing the Arduino would need to connect to GND since GND was already connected to the power supply.
Yes but you were repeatedly told that was what you needed to do.
There is a reason we speak about "electrical circuits". 
One wire to the Arduino does not a circuit make.