Click on the words "rivet_b18.jar" save the file that downloads in a folder. Next go into that folder and double click on the file after you have done that after a few seconds the program should run.
If the program doesn't run there appear to be two causes ..
1) An unzip type program has been installed which has decided that .JAR files are associated with it. If this is the case you need to change the properties of the file so it is "owned" by Java.
2) Sometimes some versions of Internet Explorer decide when downloading .JAR to automatically unpack the contents. I'm not sure how to fix this other than suggesting you use the Firefox or Chrome web browsers.