First of all you need to write and compile program using Keil. Steps for this are mentioned here. Once you have done that, let's proceed further.
Open the Proteus software from the directory where you have installed Proteus. Home screen will appear as shown.
Click on "New Project". New Project Wizard will pop-up as a result. Provide name of project and location where you want to create the new project.
Select layout for schematic design where we can add our components. For basic applications, A4 will be enough. You can select layout as per your requirement.
At present, we don't want to create PCB layout so select option accordingly.
Select following configurations for ARM7 as shown in image below.
Controller -> LPC2138
Compiler -> Keil for ARM
Verify all the information and click on Finish to create the project.
Schematic with selected micro-controller will appear as shown below.
Click on place part and search for the required component. Here we have used LED of Yellow color for demo.
To insert ground in the circuit, go for Right Click -> Place -> Terminal -> Ground.
Connect the circuit as per requirement and save it.
Double click on micro-controller. From the options click on Browse icon in program file.
Browse the hex file which we want to simulate on this schematic.
Click on PLAY icon (Triangle on Left Side Bottom) of the screen and observe the output.