1) Have you downloaded JDK 8 (http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html) ?
2) Assuming you are on a 64-bit architecture computer, ensure Java is installed here (unless specifically changed installation path): C:\Program Files\Java
3) When opening the jar file, click "open with" and navigate to C:\Program Files\Java\jdk1.8.0_152\bin
4) Open the jar file with javaw.exe.