There are two different type of downloaded simulations:
1. ZIP file: This is for simulations created before 2001 using JDK1.0.2: You should unzip the file and open index.html with your browser. (There is no jar file, but class files. )
2. JAR file: This is for simulations created after 2001 using EJS.
You can double click jar file to run it locally.

You will get the instruction if you ask our server to send file to your email account (first option).
There is no information if you download it directly (second option).