From the Plaza, exit at the southwest corner (in front of Socorro Office Supply) and turn right on Garfield, which is a one-way street going west. La Pasadita is a few blocks down on the left. No separate smoking section.
This is one of those legendary places that only the locals know about. The building is not fancy, but the food is solid, tasty and quite cost-effective. I trust their kitchen, too; I've heard extremely few reports of people having problems with La Pa's food, compared to some other local places that are quite popular.
Order at the counter, sit down, and they'll bring you the food. The word for this place is Authentic. The menu is fairly long, with all the usual standards done well: tacos, burritos, enchiladas, and so on. I like their empanadas and chimichangas, and their sopaipillas are the closest I can find to those of the late, lamented La Casita (a legendary local place of the 60s-80s), with a light, flaky texture. The burrito plate gets my vote for the most cost-effective lunch in town, and you need a pretty fair appetite to get through their combination plates.