Ah, ok, got it. Well, couple of things to check, then.
First, if you'd previously configured this Express for some other purpose, then I suppose performing a factory reset on it would be worth a try. How you do that depends on the exact model, but it'll get things back to the factory config, so you'd be certain that at least it should be discoverable.
Second, since the first-gen is a g-type only, you'd want to make sure that the Extreme is set to talk as compatible with a g network as well. Out of the box, that's how it is, but you can set it to be only n compatible (i.e., using only the 5Ghz band), and if that was the case, then the Express wouldn't be able to see the Extreme.