What do you use to run the .jar file?
If you can't just double click it/execute it/run it
It is used by Java, which most people have installed for other reasons than MLBS - JAR is a Java archive and while not all of them launch their own applet, some, and these ones, do.
I think in the event that it doesn't automatically open on its own, it means Java isn't pathe/ pathed correctly - which doesn't stop Java itself from running, but does prevent a decent amount of discovery.
You can try and just tell Java to run it, or you might try looking up how to path Java so that you can just run it without telling Java to do it.
Or you can try and wait for Slipcor to take advice on the matter for the fixer.. I feel like I remember seeing a comment somewhere talking about using it in alternative ways...