Old Weimann center-pulls are really only ajustable by taking the spring out and bending it for more tension. Another person was mistaken when he said there were screws (that's a cantilever brake that has those, not a center-pull), so ignore that input. You do, however, need cable hangars for these brakes.
If your bike came with calipers, you should stick with calipers. They weigh less, are easier to fix, and really are easier to modulate. Some people say that center-pulls give you more stopping power (probably true), but they sure aren't easy to modulate!