You're looking for the SG3524 Proteus library download. Here's some relevant text:
However, many electronics hobbyists face a common bottleneck: The default component library in Proteus Design Suite (Version 8, 9, or even the latest Proteus 8 Professional) does not include the SG3524 model.
Proteus is famous for its mixed-mode simulation (SPICE) capabilities. While it contains thousands of components, some power management ICs fall through the cracks. Without the SG3524 library, you are left with two poor options: designing without simulation (risky) or substituting a different IC (inaccurate).
The Engineering Projects: A common hub for custom Proteus Libraries that often features power electronic components.
Cause: Missing timing components (Ct and Rt). Fix:
Once you have downloaded the .LIB, .IDX, and (optionally) .HEX or .DLL files, follow these steps to integrate them: how to create aSG3524 in proteus 7 professional