Double-click the placed sensor. In the Edit Component window, find the Program File field. Click the folder icon and select the MQ135.HEX file you downloaded earlier. Simulation Pins: VCC/GND: Connect to power and ground. OUT: Connect to your microcontroller's analog pin.
Follow these steps to make the sensor appear in your Proteus component list: mq135 gas sensor library for proteus download upd work
Several community-contributed libraries are available for download: Double-click the placed sensor