By default, Proteus Isis comes with a wide range of components, but it often lacks specific modern switching regulators. Without the library, you would have to build the internal circuitry of the LM2596 from scratch using comparators and mosfets—a nightmare for simulation speed.
If you cannot get the LM2596 library to work, use the LM2576 from the default Proteus library as a temporary substitute – the simulation behavior is 90% identical.
Here is a breakdown of the top useful features and how to use them: