There are two different types of downloaded file at this website.
1. For those simulations under "jdk1.0.2 simulations" -- simulations I have created before 2001.
 The download file is [b]ZIP[/b] file. You should [u]unzip the file and open xxx.html file[/u] (where xxx ussally is the same as the filename of the ZIP file).

2. For those simulations under "Easy java simulation" -- simulations I have created since 2001-.
 The download file is [b]JAR[/b] file. Normally, you can [u]double click the JAR file[/u] to run the simulation as a java program (application).
 However, your computer should support java (i.e. installed JAVA RUN TIME or JAVA SDK).
However, if you have installed some UNZIP program, e.g. WINRAR ...etc
Those program might have set JAR program to be opened by UNZIP program,
 so window file manager open the JAR file as ZIP file ,
 then you would not be able to run it (because you have associate JAR file to UNZIP program).

You need to change the association back to [b]JAVA[/b]
The easiest way is to download the latest [url=http://www.java.com/getjava/]java run time[/url] and install it again
(or you can change the association by yourself if you know how to do it.)