An alkene has a p electron cloud and therefore usually repels nucleophiles. However, the presence of the electron-withdrawing ester group reduces the p electron denisty, and can stabilize the intermediate carbanion by resonance. A combination of these two synergystic factors allows methanethiolate to add to the p bond. The alkene shown in the example above is not electron-poor, nor are there any subsituents to provide stability for an intermediate carbanion. Therefore this alkene cannot undergo conjugate addition.
Amines can be both acids and nucleophiles.