You're opening Pong.jar file inside archive without extracting to directory, am I right?
You must extract "Pong.jar" file and "sprites" folder to one directory and then open "Pong.jar" from selected directory.
There's a line in rules which says:
"Your game must have a Linux build".
And I've got question: Can I build game to Java Bytecode, which will run under JVM on Linux? Is it allowed?