Addding Chip Schematics to Proteus
As far as I know, Proteus is the only electric simulator where you can place a micro-controller, load a compiled program, connect other electrical components to the ports, and see how the system behaves.

Despite being quite frown upon, I chose this way to learn and design my first microcontrolled circuits. And I'm still using it. Because:


Building a circuit in the simulator is like 100x times easier and faster than in real life.
You can use the specific compilation tools for the microcontroller, just like in real life.
In general I found that the digital circuits designed in this way behave quite as in the simulation.
Even when analogic or power electronics are involved, simulation gives you hints on the difficult problems.
Anyway Proteus is a great tool to make PCB printing.


Proteus needs to know three aspects of every component:


Its schematic representation, shown when drawing the schematic. The most important part of it are the number of connectors and their names.
Its package representation, shown when drawing the PCB. It contains its outer dimensions, the sizes and separations of its connectors, and the relation to its schematic representation.
Its electrical model, used during the simulation. It describes how the component behaves in term of electrical inputs and outputs.


In Proteus you can edit the first two aspects, but the third is out of your reach. Electrical models of MCU are provided by the manufacturers to Proteus. If the one you need does not exist, then you will not be able to simulate it.

Facing this problem, my advice are:


Choose a MCU that you can buy AND does exist in Proteus.
Choose a MCU that you can buy AND a similar MCU exist in Proteus.
Often a manufacturer offers lots of variations of the same MCU. For example Microchip sells PIC18F23K22, PIC18F24K22, PIC18F25K22, PIC18F26K22 which only differ in the amount of available memory.
Think 1 again. MCU are not very expensive.