I did the same thing! got a bike for christmas, and today bought that same exact computer for $19.99. At first it wasn't working either. Even if it is lined up near it, you have to first make sure the battery cover is facing out, and on other side, you will see a little arrow or something that says sensor i think, you need to put the spoke part right there. Mine was half an inch away, too low and it wouldn't registrar. So get it as close to that little arrow as you can.
Also, I put about 15 miles on it, and about 15 minutes ago as i was pulling in from a short cold ride, I noticed it wasn't giving accurate readings. It was staying on 10mph even though I was stopped. it may have moved alittle, and I will see if that fixxed it.