I am supposing that you are using keil , so you need to create a HEX file to dump in the 8051 microcontroller .
Creating a HEX file : if you have made a project see in that folder if there is a HEX file (after compling ur code) or not , if there is no HEX file , then go to flash -> configure flash tools -> output -> check the create HEX file , and recompile it you will get the hex file in the same folder
Now open proteus and make the design which will include the required microcontroller and the double click on the microcontoller and then include that hex file which was created . This way you can simulate your design .