Just squeeze hard on the Avid Single Digit 3 (?) brake-arms and pull the noodle forward. It should drop in. Look at it closely so you can see where it is supposed to go in - the end near the rubber-thingy is thinner and is meant to go into the clasp on the retainer. Failing this - carefully loosen your cable and give it a bit of slack, insert the noodle, then tighten up the cable again. Don't get it so tight that you can't remove the noodle again when needed (removing wheels, etc). You can tighten up the brakes at the levers by un-screwing the wheel that comes out with the cable in the lever.
I have a FX 7.5, so I'm rather adept with those brakes, assuming you have the Avid SD 3's. Enjoy your 7.3 - FX's are very nice bikes.