ISIS Proteus - Simulating a buzzer using an Atmega 8535 Micro controller
Insert to the schematic a buzzer (from active library), then double click the buzzer to open the properties page and set the voltage you want it to operate in, the frequency it should generate (500HZ is fine actually) and the resistance.
Ideally you want these to match the ones of the real device you want to use but for testing purposes you can set them at will.

The buzzer needs just a DC input voltage to activate, the default is 12v but if you want to feed it with 5v from a pin of the mcu then you can set it in the properties to 5v.