Here are the easy steps to add Library in Proteus
Step 1: Download the ZIP file from the Proteus library Example ARDUINO Proteus library
Step 2: Unzip the library folder. You will see two file names like * .IDX and * .LIB in some cases the third file is * .HEX.
In our example it is ARDUINO.IDX and ARDUINO.LIB
Step 3: Copy and paste these files into the LIBRARY folder of Proteus
If you are using Proteus 7 Professional, the link in the library folder will look something like this: "C: \ Program Files (x86) \ Labcenter Electronics \ Proteus 7 Professional \ LIBRARY".
If you are using Proteus 8 Professional, then the link in the library folder will look something like this: "C: \ Program Files \ Labcenter Electronics \ Proteus 8 Professional \ Data \ LIBRARY"
Step 4: Close Proteus Software and reopen it to check it.