With the leg assemblies completed and the beam notched, it's time to put the stand together. Slide each leg assembly onto the beam and into the notches as shown in the picture on this page. The cross-braces of each leg assembly should fit snugly (but not too snugly) into each pair of notches on the beam. I find it useful to wiggle the leg assembly a bit as I'm sliding the legs onto (or off of) the beam.
With both legs connected to the beam, flip the unit over onto its legs. The leg-and-beam assembly should be sturdy and have no wobble. You should be able to easily lean onto the beam or push it in any direction and it should be stable (provided that the cross-braces on the leg assemblies are properly seated in each of the notches).